void setup() {
  Serial.begin(9600);
  Serial.println("let's start serial monitor");
}

void loop() {
  // Assumes a string in from the serial port like so:
  // s ledNumber, brightness \n
  // for example: "s5,200\n":

  int ledNumber = 0;
  int brightness = 0;
  
  if (Serial.find("s")) {
    ledNumber = Serial.parseInt(); // parses numeric characters before the comma
    brightness = Serial.parseInt();// parses numeric characters after the comma
    
    // print the results back to the sender:
    Serial.print("LED number: " );
    Serial.print(ledNumber);
    Serial.print(" at ");
    Serial.println(brightness);

    // set the LED:
    analogWrite(ledNumber, brightness);
  }
}