void setup()
{
    for(int i=2; i<=13; i=i+1) {
        pinMode(i, OUTPUT);
    }
}

void loop()
{
  for(int i=2; i<=13; i=i+1) {
    digitalWrite(i, HIGH);
    delay(500);
    digitalWrite(i, LOW);
  }
  for(int i=12; i>=3; i=i-1) {
    digitalWrite(i, HIGH);
    delay(500);
    digitalWrite(i, LOW);
  }
}