void setup() {
   for (int led = 12; led < 16; led++) {
    pinMode(led, OUTPUT);
   }
   pinMode(18, INPUT); 
}

void loop() {
  int aa = digitalRead(18);
  if (aa==1) {
    for (int led = 12; led < 15; led++) {
      digitalWrite(led, HIGH);
      delay(500);
      digitalWrite(led, LOW);
    }

    for (int led = 15; led > 12; led--) {
      digitalWrite(led, HIGH);
      delay(500);
      digitalWrite(led, LOW);
    }
  }
}