const int ledPin = 13;
int delayTime = 500;

void setup() {
  pinMode(ledPin, OUTPUT);

}

void loop() {

   digitalWrite(ledPin, HIGH);

   delay(delayTime);

   digitalWrite(ledPin, LOW);

    delay(delayTime);

    delayTime -= 50;

    if (delayTime < 100) {

       delayTime = 500;
    }

}