int del = 100; //defining delay value (0.1 second)
void setup() {
  // put your setup code here, to run once:
  pinMode(2, OUTPUT); //defining pins (2-13) as Outputs
  pinMode(3, OUTPUT);
  pinMode(4, OUTPUT);
  pinMode(5, OUTPUT);
  pinMode(6, OUTPUT);
  pinMode(7, OUTPUT);
  pinMode(8, OUTPUT);
  pinMode(9, OUTPUT);
  pinMode(10, OUTPUT);
  pinMode(11, OUTPUT);
  pinMode(12, OUTPUT);
  pinMode(13, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  for (int i = 2; i <= 13; i++) { // For loop will count from 2 to 13 to turn ON and OFF every single pin
    digitalWrite(i, HIGH);        // Turning ON 1st output (LED)
    delay(del);                   // LED is ON for 20ms before turning OFF
    digitalWrite(i, LOW);         // Turning OFF 1st output (LED)
    delay(del);                   // Waiting 20ms before running the 2nd cycle of FOR loop
    }
  for (int i = 13; i >= 2; i--) { // For loop will count from 13 to 2 to turn ON and OFF every single pin
    digitalWrite(i, HIGH);        // Turning ON 1st output (LED)
    delay(del);                   // LED is ON for 20ms before turning OFF
    digitalWrite(i, LOW);         // Turning OFF 1st output (LED)
    delay(del);                   // Waiting 20ms before running the 2nd cycle of FOR loop
    }
}