const int NUM_LEDS = 9;
const int LED_PINS[] = {10, 9, 8, 7, 6, 5, 4, 3, 2};

void setup() {
  for (int i = 0; i < NUM_LEDS; i++) {
    pinMode(LED_PINS[i], OUTPUT);
  }
}

void loop() {
  for (int i = 0; i < NUM_LEDS; i++) {
    digitalWrite(LED_PINS[i], HIGH);
    delay(500);
    digitalWrite(LED_PINS[i], LOW);
  }
}