Are you able to solve it? (from the teenagers programming contest)

The official contest is over but you can try out here: <a href="http://www.spoj.pl/HSPLARCH/problems/HS11CNSM/" rel="nofollow">http://www.spoj.pl/HSPLARCH/problems/HS11CNSM/</a>

Seems like this is something like P = ((N * M) - (N + M - 1))/K but haven't tried out too many cases.