#define LED 3
#define BUTTON 4

void setup() {
  pinMode(LED,OUTPUT);
  pinMode(BUTTON,INPUT); //Define pin D2 as an input -> high impedance
}

void loop() {
  if (digitalRead(BUTTON)==HIGH){ //Read the current state of the 
                                  //BUTTON pin
    digitalWrite(LED,HIGH);
  }
  else{
    digitalWrite(LED,LOW);
  }
}