const int BUTTON_PIN = 4;
const int LED_PIN = 13;

int ledState = LOW; // initially LED is off
int lastButtonState = HIGH; // initially button is not pressed

void setup() {
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  pinMode(LED_PIN, OUTPUT);
  digitalWrite(LED_PIN, ledState);
}

void loop() {
  int buttonState = digitalRead(BUTTON_PIN);
  if (buttonState != lastButtonState) {
    // button state has changed
    if (buttonState == LOW) {
      // button is pressed
      ledState = !ledState; // toggle LED state
      digitalWrite(LED_PIN, ledState);
    }

    delay(50); 
  }

  lastButtonState = buttonState;
}