const int potPin = A0;
const int buttonPin = 13;
const int forward1 = 5;
const int forward2 = 4;
const int backward1 = 3;
const int backward2 = 2;
int potValue = 0;
int motorValue = 0;
int buttonState = 0;

void setup()
{

 pinMode(buttonPin, INPUT);
 pinMode (forward1, OUTPUT);
 pinMode (forward2, OUTPUT);
 pinMode (backward1, OUTPUT);
 pinMode (backward2, OUTPUT);

}

void loop()
{

 potValue = analogRead(potPin);
 motorValue = map(potValue, 0, 1023, 0, 255);
 buttonState = digitalRead(buttonPin);

 if (buttonState == LOW)
  {

   analogWrite(backward1, motorValue);
   digitalWrite (backward2, HIGH);
   digitalWrite (forward1, LOW);
   digitalWrite (forward2, LOW);

  }

 else
  {

   analogWrite(forward1, motorValue);
   digitalWrite (forward2, HIGH);
   digitalWrite (backward1, LOW);
   digitalWrite (backward2, LOW);

  }

}