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