//INPUT_PULLUP by default is HIGH. 
//On grounding the pin, the value changes to low
//by default returning to HIGH on release

void setup() {
  // put your setup code here, to run once:
  pinMode(2, OUTPUT);
  pinMode(13, INPUT_PULLUP);
}

void loop() {
  // put your main code here, to run repeatedly:
  digitalWrite(2, !digitalRead(13));
}