void setup() {
    Serial.begin(115200);
    
    bool flag;
    while(millis()/1000 <= 5){
      digitalWrite(LED_BUILTIN, (millis()/90)%2);

      if( (millis()/500)%2 ){
        if( flag ){
          Serial.println(millis()/1000);
          flag = false;
        }
      }else{
        flag = true;
      }

    }

}

void loop() {
  // put your main code here, to run repeatedly:

}