#define led1 2
#define led2 4

void setup() {
  pinMode(led1, OUTPUT);
  pinMode(led2, OUTPUT);
}

void loop() {
  digitalWrite(led1, HIGH);
  digitalWrite(led2, LOW);
  delay(1000); // this speeds up the simulation
  digitalWrite(led1, LOW);
  digitalWrite(led2, HIGH);
  delay(1000);
}