void setup()
{
  for (int p = 12; p<= 15; p++)
  {
    pinMode(p, OUTPUT);
  }
}
void loop()
{
  for(int y= 12 ; y <= 15; y=y+1)
  {
    digitalWrite(y, HIGH);
    delay(100);
    digitalWrite(y, LOW);
  }
   for(int y= 15; y >= 12; y=y-1)
  {
    digitalWrite(y, HIGH);
    delay(100);
    digitalWrite(y, LOW);
  }
}