int d = 250;

void setup() {
  pinMode(2, OUTPUT);
  pinMode(3, OUTPUT);
  pinMode(4, OUTPUT);
  pinMode(5, OUTPUT);
  pinMode(6, OUTPUT);
}

void loop() {
  for (int a = 2; a < 7; a++) {
    digitalWrite(a, HIGH);
    delay(d);
    digitalWrite(a, LOW);
    delay(d);
  }

  for (int a = 5; a > 1; a--) {
    digitalWrite(a, HIGH);
    delay(d);
    digitalWrite(a, LOW);
    delay(d);
  }
}