void setup() { // initialize digital pin 2 as an output.
  pinMode(2, OUTPUT);
  pinMode(13, OUTPUT);
}
// the loop function runs over and over again forever
void loop() {
  digitalWrite(13, HIGH); // turn the LED1 on (HIGH is the voltage level)
  digitalWrite(2, LOW); //turn the LED2 off
  delay(1000); // wait for a second
  digitalWrite(13, LOW); // turn the LED1 off by making the voltage LOW
  digitalWrite(2, HIGH);// turn the LED2 on
  delay(1000); // wait for a second
}