void setup (){
  pinMode(18, OUTPUT);
  pinMode(25, OUTPUT);
  pinMode(27, OUTPUT);
  pinMode(23, OUTPUT);
}

void loop (){

  digitalWrite(18, HIGH);
  digitalWrite(25, LOW);
  digitalWrite(27, HIGH );
  digitalWrite(23, LOW);
  delay(2000);

  digitalWrite(18, LOW);
  digitalWrite(25, HIGH);
  digitalWrite(27, LOW);
  digitalWrite(23, HIGH);
  delay(2000);
  
}