Gunnar and his friends like games which involve rolling dice. Gunnar has a huge collection of 6-sided, 12-sided and 20-sided dice. All the games with dice started to bore him, so he came up with a new game. He rolls an $s$-sided die $n$ times and wins if at least $k$ different numbers appear in the $n$ throws. An $s$-sided die contains $s$ distinct numbers $1, \dots, s$ on its sides.
Since this is a game only for one person, Gunnar and his friends decided to make it more fun by letting other people bet on a particular game. Before you bet on a particular game, you would like to know how probable it is to throw at least $k$ different numbers in $n$ throws with an $s$-sided die. We assume that all numbers have the same probability of being thrown in each throw.