Billiard balls and the number theory result

Hello,

"Given a rectangular billiard table with only corner pockets and sides of integer lengths m and n (with m and n relatively prime), a ball sent at a 45 degree angle from a corner will be pocketed in another corner after m+n-2 bounces ".

This is the Steinhaus and Gardner result.

Can anyone please tell were i can find (on the Internet = for free) the proof of this result.

i could find the books published by this authors on ebay and similar websites but all i need a free version as i can not effort any of them.

Thanks in advance,

Pitagora's