volatile unsigned int count = 0;
volatile int flag = 0;

void setup()
{
  Serial.begin(38400);
  attachInterrupt(0, isr, FALLING);  //INTERRUPT 0 IS PIN D2
}

void loop()
{
  if (flag == 1)
  {
  Serial.println(count);
  flag = 0;
  }
}

void isr()
{
  count += 1;
  flag = 1;
}