int BUTTON=4;
volatile byte state = LOW;
void setup()
{
  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(BUTTON, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(BUTTON), blink,RISING);
}
void loop() {
  digitalWrite(LED_BUILTIN, state);
}
void blink() {
  state = !state;
}