int R1 = 16;
int R2 = 15;
int LEDPinR1 = 13;
int LEDPinR2 = 12;

void setup() {

  Serial.begin(9600);
  pinMode(LEDPinR1, OUTPUT);
  pinMode(LEDPinR2, OUTPUT);

}

void loop() {

  if (R1 > R2) {

    int Rgr = R1 + R2;
    Serial.print("Ergebnis der Reihenschaltung");
    Serial.print("= ");
    Serial.println(Rgr);
    digitalWrite(LEDPinR1, HIGH);
    digitalWrite(LEDPinR2, LOW);
    delay(1000);

  }
  else {
    double Rgp1 = R1 * R2;
    double Rgp2 = R1 + R2;
    double Rgp = Rgp1/Rgp2;

    Serial.println("Ergebnis der Parallelschaltung");
    Serial.print("= ");
    Serial.println(Rgp);
    digitalWrite(LEDPinR1, LOW);
    digitalWrite(LEDPinR2, HIGH);
    delay(1000);
  }
}