int led = 2;      
int stop = 6;
const byte pin = 21;
int potPin = A0;
int potVal = 0;


void setup() {
  pinMode(led, OUTPUT);
  attachInterrupt(digitalPinToInterrupt(pin), kassadin, HIGH);
}

void loop() {
  potVal = analogRead(potPin);    
  int delayTime = map(potVal, 0, 1023, 0, 10); 
  delay(delayTime);

  for (int i = 0; i <= 255; i++) {
    analogWrite(led, i);
    analogWrite(led - 1, 255 - i);
    delay(delayTime);
  }

  led++;
  digitalWrite(led, HIGH);
  if (led >= stop) {
    led = 2;
    stop--;
  }
}

void kassadin() {
  for (int i = 2; i <= 6; i++) {
    digitalWrite(i, LOW);
    led = 2;
    stop = 6;
  }
}