#define led1 15
#define led2 2
#define btn 4

int btn_state;
int btn_currState;
int led_state = LOW;

void setup() {
  Serial.begin(9600);
  pinMode(led1, OUTPUT);
  pinMode(led2, OUTPUT);
  pinMode(btn, INPUT_PULLUP);

  btn_state = digitalRead(btn);
}

void loop() {
  btn_currState = btn_state;
  btn_state = digitalRead(btn);

  if(btn_currState == HIGH) {
    Serial.println("led1 active");
    led_state = !led_state;

    digitalWrite(led1, led_state);
    digitalWrite(led2, LOW);
  } else {
    Serial.println("led2 active");
    led_state = !led_state;

    digitalWrite(led1, LOW);
    digitalWrite(led2, led_state);
  }
  delay(500);
}