#define PIN_DECREASE 1
#define PIN_INCREASE 2

int16_t counter = 0;
int8_t old_state_pin_decrease = 0;
int8_t old_state_pin_increase = 0;
int8_t current_state_pin_decrease = 0;
int8_t current_state_pin_increase = 0;

void setup()
{
  pinMode(PIN_DECREASE, INPUT);
  pinMode(PIN_INCREASE, INPUT);
}

void loop()
{
   current_state_pin_decrease = digitalRead(PIN_DECREASE);
   if (current_state_pin_decrease != old_state_pin_decrease)
   {
      counter--;
      old_state_pin_decrease = current_state_pin_decrease;
   }

   current_state_pin_increase = digitalRead(PIN_INCREASE);
   if (current_state_pin_increase != old_state_pin_increase)
   {
      counter++;
      old_state_pin_increase = current_state_pin_increase;
   }

}