// constants won't change
const int buttonPin = 2; 
const int ledPin    = 21; 
int buttonState = 0;    
void setup() {
  pinMode(ledPin, OUTPUT);
  pinMode(buttonPin, INPUT_PULLUP);
}
void loop() {
 buttonState = digitalRead (buttonPin);
 if (buttonState == HIGH) {
    digitalWrite(ledPin, LOW);
} else {
    digitalWrite(ledPin, HIGH);
 }
}