// Press the button to stop blinking the LED!

long int counter = 0;

void isr() {
  counter++;
}

void setup() {
  Serial.begin(115200);
  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(2, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), isr, LOW);
}

void loop() {
  digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
  delay(250);
  if (counter) {
    Serial.print(F("Interrupt fired "));
    Serial.print(counter);
    Serial.println(F(" times!"));
    counter = 0;
  }
}