Compute number of integers divisible by $k$ in range $[a..b]$.
given three integers $A, B$ and $K$, returns the number of integers within the range $[A..B]$ that are divisible by $K$, i.e.: $\{ i : A ≤ i ≤ B, i \bmod K = 0 \}$
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 ≤ B.

returns the number of integers within the range $[A..B]$ that are divisible by $K$.