const int buttonPin = 2;
const int ledPin = 13;

void setup() {
  //enable pull-up resistor
  pinMode(buttonPin, INPUT_PULLUP);
  pinMode(ledPin, OUTPUT);
}

void loop() {
  int buttonVal = digitalRead(buttonPin);

  if (buttonVal == HIGH) {
    digitalWrite(ledPin, LOW);
  } else {
    digitalWrite(ledPin, HIGH);
  }
  delay(100);
}