int a = 500;
void setup() {
  Serial.begin(9600);
  pinMode(12, INPUT_PULLUP);
  pinMode(13, OUTPUT);


  if(digitalRead(12) == LOW)
  {
      Serial.println("Napis rychlost");
      while (!Serial.available());
      int a = Serial.parseInt();
      Serial.println("Aktualna rychlost : "  + String(a));

  }


}

void loop() {
digitalWrite(13, HIGH);
delay(a);
digitalWrite(13, LOW);
delay(a);
}