2D Collisions II

On an infinite two-dimensional frictionless plane, $m$ unit squares slide in the positive direction, while $n$ more unit square slide towards them in the other direction. When the unit squares collide, they will elastically rebound in the other direction with the same speed. Let $x$ be the number of squares that end up proceeding in the negative direction, while $y$ be the number of squares that end up proceeding in the positive direction. Find the value of $y-x$ when $m = 100$ and $n = 50$.