// E4.4
const int ledPin = 3;
void setup() {
    pinMode(ledPin, OUTPUT);
    Serial.begin(9600);
}
void loop() {
   byte brightness; 
   if (Serial.available() > 0) {
       brightness = Serial.parseInt();
       brightness = constrain(brightness, 0, 255);
       analogWrite(ledPin, brightness);
   }
}