void setup() {
  for(int i=2;i<=6;i+=2){
    pinMode(i, OUTPUT);
  }
}

void loop() {
  for(int i=2;i<=6;i+=2){
    digitalWrite(i, HIGH);
    delay(100);
    digitalWrite(i, LOW);
    delay(100);
  }
  for(int i=6;i>=2;i-=2){
    digitalWrite(i, HIGH);
    delay(100);
    digitalWrite(i, LOW);
    delay(100);
  }
}