#include <Wire.h>
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 16, 2);

#define button 6
#define greenled 8
#define redled 9

unsigned long startTime = 0;
unsigned long elapsedTime = 0;
bool counting = false;

void setup() {
  
  lcd.init();
  lcd.backlight();
  pinMode(greenled, OUTPUT);
  pinMode(redled, OUTPUT);
  pinMode(button, INPUT_PULLUP);

  lcd.setCursor(0, 0);
  lcd.print("   STOPWATCH   ");
  lcd.setCursor(0, 1);
  lcd.print("      second's ");
}

void loop() {

  if (digitalRead(button) == LOW) {
 
    counting = !counting;

    if (counting) {
      startTime = millis();
      digitalWrite(greenled, HIGH);
      digitalWrite(redled, LOW);
    }
  
    else {
      elapsedTime = millis() - startTime;
      digitalWrite(greenled, LOW);
      digitalWrite(redled, HIGH);
    }

    while (digitalRead(button) == LOW) {}
  }

  if (counting) {
    elapsedTime = millis() - startTime;
    lcd.setCursor(0, 1);
    lcd.print(elapsedTime / 1000);
    lcd.print(".");
    lcd.print(elapsedTime % 1000 / 100);
  }
  
  else {
    lcd.setCursor(0, 1);
    lcd.print("     ");
    lcd.setCursor(0, 1);
    lcd.print(elapsedTime / 1000);
    lcd.print(".");
    lcd.print(elapsedTime % 1000 / 100);
  }
}