const int fan = 13; // the pin that the LED is attached to
int input;     // a variable to read incoming serial data into
int cislo = 20;
char buffer[4];

void setup() {
  // initialize serial communication:
  Serial.begin(9600);
  // initialize the LED pin as an output:
  pinMode(fan, OUTPUT);
}

void loop() {
  if (cislo < 999)
  {
  digitalWrite(fan, HIGH);
  delay(20); 
  }
  digitalWrite(fan, LOW);  
  delay(cislo);
  if ( Serial.available() >= 3 )
  {
    buffer[0] = Serial.read();
    buffer[1] = Serial.read();
    buffer[2] = Serial.read();
    buffer[3] = '\0';
    cislo = atoi(buffer);
    Serial.print("I received: ");
    Serial.println(cislo, DEC);
  }
}