// Set pin 0 as an output
const int ledPin = 0;
int a = 10;

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

void loop() {
  // Make your led blink 5 times (=10/2 because toggle)
  while (a > 0) {
    a -= 1;           // equivalent to a=a-1
    digitalWrite(ledPin, !digitalRead(ledPin)); // Change LED's status
    delay(500);       // Wait for 0.5 seconds
  }
}