int led1 = 8;
int led2 = 9;

void setup() {
  pinMode(led1, OUTPUT);
  pinMode(led2, OUTPUT);
}

void loop() {
  // LED 1 ON for 500 ms
  digitalWrite(led1, HIGH);
  delay(500);
  digitalWrite(led1, LOW);

  // LED 2 ON for 500 ms
  digitalWrite(led2, HIGH);
  delay(500);
  digitalWrite(led2, LOW);
}