const int startStopButton = 4; // the pin number of the start/stop button
unsigned long startTime = 0; // the time the timer was started
unsigned long elapsedTime = 0; // the elapsed time of the timer
bool isRunning = false; // whether the timer is currently running

void setup() {
  pinMode(startStopButton, INPUT_PULLUP); // set the start/stop button as input with pull-up resistor
  Serial.begin(9600); // initialize serial communication
}

void loop() {
  if (digitalRead(startStopButton) == LOW) { // if the start/stop button is pressed
    if (isRunning) { // if the timer is running, stop it
      elapsedTime = millis() - startTime;
      isRunning = false;
    } else { // if the timer is not running, start it
      startTime = millis() - elapsedTime;
      isRunning = true;
    }
    delay(200); // debounce the button
  }

  if (isRunning) { // if the timer is running, print the elapsed time
    unsigned long currentTime = millis();
    elapsedTime = currentTime - startTime;
    Serial.print("Elapsed time: ");
    Serial.print(elapsedTime / 1000);
    Serial.println(" seconds");
  }
}