int RED = 13;
int YELLOW = 12;
int GREEN = 8;

void setup()
{
  pinMode(RED, OUTPUT);
  pinMode(YELLOW, OUTPUT);
  pinMode(GREEN, OUTPUT);
}

void loop()
{
  digitalWrite(RED, HIGH);
  delay(1000);
  digitalWrite(RED, LOW);

  digitalWrite(YELLOW, HIGH);
  delay(1000);
  digitalWrite(YELLOW, LOW);

  digitalWrite(GREEN, HIGH);
  delay(1000);
  digitalWrite(GREEN, LOW);
}