int cislo = 3;
void setup() {
 Serial.begin(9600);
  pinMode(13, OUTPUT);
  pinMode(3,INPUT);
  digitalWrite(3, HIGH);
}

void loop() {
  digitalWrite(13, HIGH);
  delay(500);
  digitalWrite(13, LOW);
  delay(500);
  
  if(digitalRead(3) == LOW)
  {
    cislo++;
    Serial.println(cislo);
  }
}