void setup() {
  for(int i = 12 ; i<=15; i++){
    pinMode(i, OUTPUT);
  }
}

void loop() 
{
  for(int i = 12; i<=15;i++){
  digitalWrite(i, HIGH);
  delay(500);
  digitalWrite(i, LOW);
  }
  for(int i = 15; i<=12;i--){
  digitalWrite(i, HIGH);
  delay(500);
  digitalWrite(i, LOW);
  }



}