volatile int counter = 0;

void setup() {
  Serial.begin(9600);
  attachInterrupt(digitalPinToInterrupt(2), increment,RISING);
  attachInterrupt(digitalPinToInterrupt(3), decrement,RISING);

}

void loop() {
  Serial.println(counter);
  delay(500);
}

void increment () {
  counter++;
}

void decrement () {
  counter--;
}