int rled = A3;
int gled = 2;

void setup() {
  pinMode(rled, OUTPUT);    // or in place of rled u can write A3
  pinMode(gled, OUTPUT);    // or in place of rled u can write 2
}

void loop() {
 digitalWrite(rled, HIGH);
 digitalWrite(gled, LOW);
 delay(1000);
 digitalWrite(rled, LOW);
 digitalWrite(gled, HIGH);
 delay(1000);
}