How far from free shipping

Problem

The store ships for free once your order reaches a certain amount. Take cart, what you have so far, and minimum, the amount that unlocks free shipping, and return how much more you need.

With 180 in the cart and a minimum of 500, you need 320 more.

If you already got there, or went past it, you need nothing: return 0, never a negative number.

Examples

• 180, 500 → 320

• Right on the minimum

  500, 500 → 0

• Empty cart

  0, 300 → 300

Besides these, the challenge has hidden tests that are revealed when you submit your solution.

You start with this

def still_needed(cart, minimum):
    pass

function stillNeeded(cart, minimum) {
}