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:
}