const int leds[] = {6, 5, 4, 3, 2, 7, 8, 9}; 
const int NUM_LED = 8; 
const int delayTime = 500; 

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

void loop() {
  for (int i = 0; i < NUM_LED; i += 2) { 
    digitalWrite(leds[i], HIGH);  
    digitalWrite(leds[i + 1], HIGH);
    delay(delayTime);
    digitalWrite(leds[i], LOW);    
    digitalWrite(leds[i + 1], LOW); 
  }
}