#define LED_RED 2
#define LED_GREEN 4

void setup() {
  pinMode(LED_RED, OUTPUT);
  pinMode(LED_GREEN, OUTPUT);
}

void loop() {
  digitalWrite(LED_RED, HIGH);
  digitalWrite(LED_GREEN, LOW);
  delay(500);
  digitalWrite(LED_RED, LOW);
  digitalWrite(LED_GREEN, HIGH);
  delay(500);
}