int LED_inBUILT = PC13; 

// the setup function runs once when you press reset or power the board
void setup() {
  // initialize digital pin LED_inBUILT as an output.
  pinMode(LED_inBUILT, OUTPUT);
}

// the loop function runs over and over again forever
void loop() {
  digitalWrite(LED_inBUILT, HIGH);  // turn the LED on (HIGH is the voltage level)
  delay(5000);                      // wait for a second
  digitalWrite(LED_inBUILT, LOW);   // turn the LED off by making the voltage LOW
  delay(5000);                      // wait for a second
}