const uint8_t RED_PIN = 9;
const uint8_t GREEN_PIN = 10;

uint8_t brightness = 0;
uint8_t fade_amount = 5;

void setup() {
  pinMode(RED_PIN, OUTPUT);
  pinMode(GREEN_PIN, OUTPUT);

  digitalWrite(RED_PIN, HIGH);
}

void loop() {
  analogWrite(GREEN_PIN, brightness);

  brightness = brightness + fade_amount;

  if (brightness <= 0 || brightness >= 255) {
    fade_amount = -fade_amount;
  }

  delay(30);
}