hit.c

int pinHit = 2;
int count = 0;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  pinMode(pinHit, INPUT);

  attachInterrupt(0, hitISR, FALLING);
}

void loop() {
  // put your main code here, to run repeatedly:
  Serial.print("HIT : ");
  Serial.println(count);
  delay(500);
}

void hitISR(void) {
  count++;
}