const int buttonPin = 4;
const int ledPin = 15;

int buttonState = 0;
int lastButtonState = 0;
int ledState = LOW;

void setup() {
  // put your setup code here, to run once:
  pinMode(ledPin, OUTPUT);
  pinMode(buttonPin, INPUT);
}

void loop() {
  
  buttonState = digitalRead(buttonPin);

  if(buttonState == HIGH && lastButtonState == LOW){
    if(ledState == LOW){
      ledState = HIGH;
    } else {
      ledState = LOW;
    }
    digitalWrite(ledPin, ledState);
  }

  lastButtonState = buttonState;
}