int led1_3 = 2;
int led2_4 = 15;

void setup() {
  pinMode(led1_3, OUTPUT);
  pinMode(led2_4, OUTPUT);
}

void loop() {
  for (int i = 0; i <= 10000; i + 30) {
    digitalWrite(2, HIGH);
    delay(i);
    digitalWrite(2, LOW);
    delay(i);
    digitalWrite(15, HIGH);
    delay(i);
    digitalWrite(15, LOW);
    delay(i);

  }
}