int LED1 = 1;
int LED2 = 3;
int BUTTON = 2;

volatile int buttonPressed = 0;

void setup() {
  pinMode(LED1, OUTPUT);
  pinMode(LED2, OUTPUT);
  pinMode(BUTTON, INPUT_PULLUP);
  Serial.begin(9600);

  attachInterrupt(digitalPinToInterrupt(BUTTON), isrButton, FALLING);
}

void loop() {
  digitalWrite(LED1, HIGH);
  delay(500);
  digitalWrite(LED1, LOW);
  delay(500);
  Serial.println(buttonPressed);
  if (buttonPressed == 1) {
    digitalWrite(LED2, HIGH);
  } else {
    digitalWrite(LED2, LOW);
  }
}

void isrButton() {
  buttonPressed = 1;
}