Codility - CountDiv (JavaScript)

I initially kept doing this: (B-A)/K + Offset and continued wondering why it didn't work. Took me a while to realize what was up. Prefix SUM!

Task description
Write a function:
function solution(A, B, K);

that, given three integers A, B and K, returns the number of integers within the range [A..B] that are divisible by K.

For example, for A = 6, B = 11 and K = 2, your function should return 3, because there are three numbers divisible by 2 within the range [6..11], namely 6, 8 and 10.

Write an efficient algorithm for the following assumptions:

A and B are integers within the range [0..2,000,000,000];
K is an integer within the range [1..2,000,000,000];
A less than or equal to B.

[fredteixeira]

Brilliant!
This is what I did:
function solution(A, B, K) {
// calc first divisible
var a0 = A%K === 0 ? A : (A - A%K)+K;
// calc last divisible
var bn = B%K === 0 ? B : B - B%K;

var countMiddle = (bn-a0) / K;
return countMiddle+1;
}