diff --git a/Cargo.lock b/Cargo.lock
index 16ec191..04df67e 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -25,11 +25,18 @@ dependencies = [
  "avr-hal-generic",
 ]
 
+[[package]]
+name = "autocfg"
+version = "1.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ace50bade8e6234aa140d9a2f552bbee1db4d353f69b8217bc503490fc1a9f26"
+
 [[package]]
 name = "avr-calc"
 version = "0.1.0"
 dependencies = [
  "arduino-hal",
+ "calc-math",
  "embedded-hal 1.0.0",
  "nb 1.1.0",
  "proc-macro2",
@@ -81,6 +88,13 @@ version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f8fe8f5a8a398345e52358e18ff07cc17a568fbca5c6f73873d3a62056309603"
 
+[[package]]
+name = "calc-math"
+version = "0.1.0"
+dependencies = [
+ "num-traits",
+]
+
 [[package]]
 name = "cfg-if"
 version = "1.0.0"
@@ -140,6 +154,15 @@ version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8d5439c4ad607c3c23abf66de8c8bf57ba8adcd1f129e699851a6e43935d339d"
 
+[[package]]
+name = "num-traits"
+version = "0.2.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "071dfc062690e90b734c0b2273ce72ad0ffa95f0c74596bc250dcfd960262841"
+dependencies = [
+ "autocfg",
+]
+
 [[package]]
 name = "paste"
 version = "1.0.15"
diff --git a/Cargo.toml b/Cargo.toml
index db7d960..2dcceeb 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -14,6 +14,7 @@ bench = false
 ufmt = "0.2.0"
 nb = "1.1.0"
 embedded-hal = "1.0"
+calc-math = { path = "../calc-math" }
 
 [dependencies.arduino-hal]
 git = "https://github.com/rahix/avr-hal"
diff --git a/src/main.rs b/src/main.rs
index e700625..d34fc0d 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,6 +1,8 @@
 #![no_std]
 #![no_main]
 
+use calc_math::calc::StackCalc;
+use calc_math::Decimal;
 use core::ops::{Index, IndexMut};
 
 use arduino_hal::{
@@ -476,7 +478,31 @@ impl NumberInput {
         }
     }
 
-    fn apply(&self, display: &mut DispalyState) {
+    fn to_decimal(&self) -> Decimal<5, u8> {
+        Decimal {
+            minus: self.minus,
+            significant: self.significant,
+            minus_exponent: self.minus_exponent,
+            exponent: self.exponent[0] * 10 + self.exponent[1],
+        }
+    }
+
+    fn set_decimal(&mut self, dec: Decimal<5, u8>) {
+        self.minus = dec.minus;
+        self.significant = dec.significant;
+        self.minus_exponent = dec.minus_exponent;
+        self.exponent[0] = dec.exponent / 10;
+        self.exponent[1] = dec.exponent - dec.exponent / 10;
+    }
+}
+
+// Show data on segment display
+trait Show {
+    fn show(&self, display: &mut DispalyState);
+}
+
+impl Show for NumberInput {
+    fn show(&self, display: &mut DispalyState) {
         display[0] = if self.minus {
             SegmentState::Minus
         } else {
@@ -497,6 +523,13 @@ impl NumberInput {
     }
 }
 
+enum State {
+    EnterSignificant,
+    EnterExponent,
+    EnterOperation,
+    Err,
+}
+
 #[arduino_hal::entry]
 fn main() -> ! {
     let dp = arduino_hal::Peripherals::take().unwrap();
@@ -539,7 +572,10 @@ fn main() -> ! {
     let mut number_input = NumberInput::default();
 
     let mut display = DispalyState::default();
-    number_input.apply(&mut display);
+    number_input.show(&mut display);
+
+    let mut calc = StackCalc::<2, 5, u8>::default();
+    let mut state = State::EnterSignificant;
 
     loop {
         let mut key: Option<(usize, bool, bool)> = None;
@@ -558,25 +594,84 @@ fn main() -> ! {
         }
 
         let key = key.and_then(|(no, kn, ko)| InputKey::map(no, kn, ko));
-        // TODO: impl main state
-        let mut err = Ok(());
 
         if let Some(key) = debounce.input(key) {
             ufmt::uwriteln!(&mut serial, "key: {:?}", key).unwrap_infallible();
-            match key {
-                InputKey::Up => todo!(),
-                InputKey::C => number_input.reset(),
-                InputKey::Num(val) => {
-                    err = number_input.input(val);
-                }
-                InputKey::Mul => todo!(),
-                InputKey::Div => todo!(),
-                InputKey::Plus => todo!(),
-                InputKey::Minus => number_input.toggle_minus(),
-                InputKey::Down => todo!(),
-                InputKey::E => number_input.toggle_enter_exponent(),
+            let res = match state {
+                State::EnterSignificant => match key {
+                    InputKey::C => Ok(number_input.reset()),
+                    InputKey::Num(val) => number_input.input(val),
+                    InputKey::Minus => Ok(number_input.toggle_minus()),
+                    InputKey::E => {
+                        number_input.toggle_enter_exponent();
+                        state = State::EnterExponent;
+                        Ok(())
+                    }
+                    _ => Ok(()),
+                },
+                State::EnterExponent => match key {
+                    InputKey::C => {
+                        number_input.reset();
+                        state = State::EnterSignificant;
+                        Ok(())
+                    }
+                    InputKey::Num(val) => number_input.input(val),
+                    InputKey::Minus => Ok(number_input.toggle_minus()),
+                    InputKey::E => {
+                        number_input.toggle_enter_exponent();
+                        match calc.push(number_input.to_decimal()) {
+                            Ok(()) => {
+                                number_input.reset();
+                                state = State::EnterSignificant;
+                            }
+                            Err(_) => {
+                                state = State::EnterOperation;
+                            }
+                        }
+                        Ok(())
+                    }
+                    _ => Ok(()),
+                },
+                State::EnterOperation => match key {
+                    InputKey::Up => todo!(),
+                    InputKey::C => {
+                        number_input.reset();
+                        state = State::EnterSignificant;
+                        Ok(())
+                    }
+                    InputKey::Num(_) => todo!(),
+                    InputKey::Mul => todo!(),
+                    InputKey::Div => todo!(),
+                    InputKey::Plus => match calc.add() {
+                        Ok(dec) => {
+                            number_input.set_decimal(dec);
+                            Ok(())
+                        }
+                        Err(_) => Err(()),
+                    },
+                    InputKey::Minus => todo!(),
+                    InputKey::Down => todo!(),
+                    InputKey::E => todo!(),
+                },
+                State::Err => match key {
+                    InputKey::C => {
+                        number_input.reset();
+                        state = State::EnterSignificant;
+                        Ok(())
+                    }
+                    _ => Ok(()),
+                },
+            };
+            if res.is_err() {
+                state = State::Err
+            }
+
+            match state {
+                State::EnterSignificant | State::EnterExponent | State::EnterOperation => {
+                    number_input.show(&mut display)
+                }
+                State::Err => todo!("display error"),
             }
-            number_input.apply(&mut display);
         }
         arduino_hal::delay_ms(1);
     }