#include <LiquidCrystal_I2C.h>

#define I2C_ADDR    0x27
#define LCD_COLUMNS 20
#define LCD_LINES   4

byte startstop = 2;
byte reset = 3;
bool szamol = false;
unsigned long start = 0;
unsigned long eltelt = 0;
int szamlalo = 0;

LiquidCrystal_I2C lcd(I2C_ADDR, LCD_COLUMNS, LCD_LINES);

void setup() {
  pinMode(startstop, INPUT_PULLUP);
  pinMode(reset, INPUT_PULLUP);
  lcd.init();
  lcd.backlight();
  lcd.setCursor(0, 1);
  lcd.print("Eltelt ido:      (s)");
}

void loop() {
  if (digitalRead(startstop) == LOW) {
    delay(50);
    if (!szamol) {
      start = millis();
    }
    szamol = !szamol;
    while (digitalRead(startstop) == LOW) {}
  }

  if (!szamol) {
    if (digitalRead(reset) == LOW) {
      delay(50);
      szamlalo = 0;
      lcd.setCursor(13, 1);
      lcd.print("     ");
      while (digitalRead(reset) == LOW) {}
    }
  }

  if (szamol) {
    unsigned long current = millis();
    eltelt = (current - start) / 1000;
    lcd.setCursor(13, 1);
    lcd.print(eltelt);
  }

  delay(10);
}