int r1 = 4, yl1 = 2, g1 = 15;
int r2 = 21, y2 = 19, g2 = 18;
void setup() {
  pinMode(r1, OUTPUT);
  pinMode(yl1, OUTPUT);
  pinMode(g1, OUTPUT);
  pinMode(r2, OUTPUT);
  pinMode(y2, OUTPUT);
  pinMode(g2, OUTPUT);
}

void loop() {
  digitalWrite(g1, HIGH);
  digitalWrite(r2, HIGH);
  digitalWrite(yl1, LOW);
  digitalWrite(y2, LOW);
  delay(3000);

  digitalWrite(g1, LOW);
  digitalWrite(r1, LOW);
  digitalWrite(g2, LOW);
  digitalWrite(r2, LOW);
  digitalWrite(yl1, HIGH);
  digitalWrite(y2, HIGH);
  delay(2000);

  digitalWrite(yl1, LOW);
  digitalWrite(y2, LOW);
  digitalWrite(g2, HIGH);
  digitalWrite(r1, HIGH);
  delay(3000);

  digitalWrite(g1, LOW);
  digitalWrite(r1, LOW);
  digitalWrite(g2, LOW);
  digitalWrite(r2, LOW);
  digitalWrite(yl1, HIGH);
  digitalWrite(y2, HIGH);
  delay(2000);
}