static volatile int count = 0;

void setup() {
  pinMode(2, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(2), onButtonPress, FALLING);
  Serial.begin(115200);
}

void loop() {
  Serial.print("Count = ");
  Serial.println(count);
  delay(1000);
}

void onButtonPress() {
  count += 1;
}