const int led[] = {1,6,16,20};
const int leds = 4;

void setup() {
  // put your setup code here, to run once:
  for(int i = 0;i<leds;i++)
   pinMode(led[i], OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  for(int i = 0;i<leds;i++)
   { digitalWrite(led[i], HIGH);
    delay(500); 
    digitalWrite(led[i], LOW);
   }
  
   for(int i = leds - 1;i >= 0;i--)
   { digitalWrite(led[i], HIGH);
    delay(500); 
    digitalWrite(led[i], LOW);
   }

}