int ld = 13;

void setup() {
  Serial.begin(9600);
  pinMode(ld, OUTPUT);
}

int state() {
  if (Serial.available() > 0) {
    String s = Serial.readString();
    if (s == "L" || s == "l") {
      return 1;
    }
    else if (s == "D" || s == "d") {
      return 2;
    }
    else {
      return 3;
    }
  }
  else {
    return 0;
  }
}

void loop() {
  int currState = state();
  Serial.println(currState);
  if (currState == 1) { digitalWrite(ld, 1); }
  else if (currState == 2) { digitalWrite(ld, 0); }
  else if (currState == 3) { Serial.println("invalid command"); }
  else { Serial.println("error"); }
}