void setup() {
  // initialize digital pin LED_BUILTIN as an output.
   pinMode(18, OUTPUT);
   pinMode(16, OUTPUT);
   pinMode(15, OUTPUT);
}

// the loop function runs over and over again forever
void loop() {
  digitalWrite(18, HIGH);  // turn the LED on (HIGH is the voltage level)
  delay(100);                      // wait for a second
  digitalWrite(18, LOW);   // turn the LED off by making the voltage LOW
  delay(100);
  digitalWrite(16, HIGH);  // turn the LED on (HIGH is the voltage level)
  delay(100); 
  digitalWrite(16, LOW);  // turn the LED on (HIGH is the voltage level)
  delay(100); 
  digitalWrite(15, HIGH);  // turn the LED on (HIGH is the voltage level)
  delay(100); 
  digitalWrite(15, LOW);  // turn the LED on (HIGH is the voltage level)
  delay(100);   
}
                                                                                      // wait for a second