int cnt = 0;
int pinInput = 6;
int pinOutput = 8;
bool posledno = false;

void setup() {
  // put your setup code here, to run once:
  pinMode(pinInput, INPUT);
  pinMode(pinOutput, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  bool sea = digitalRead(pinInput);
  if (sea != posledno) {
    if (sea == false) cnt++;

    posledno = sea;
  }

  if (cnt == 20) {
    digitalWrite(pinOutput, HIGH);
    delay(500);
    digitalWrite(pinOutput, LOW);
    cnt = 0;
  }

  delay(2);
}