KöMaL 2012 B4486

Let $a$ and $b$ be positive integers. How many non-negative integers $n$ are there such that $\left[\frac{n}{ab}\right] + \left[\frac{n+b}{ab}\right] + \left[\frac{n+2b}{ab}\right] + \ldots + \left[\frac{n+(a-1)b}{ab}\right] = \left[\frac{n}{ab}\right] + \left[\frac{n+a}{ab}\right] + \left[\frac{n+2a}{ab}\right] + \ldots + \left[\frac{n+(b-1)a}{ab}\right]$? ([$x$] denotes the greatest integer not greater than $x$.)