Bob is playing with a chess board!
Bob currently has a \(1 \times N\) board as well as \(M\) king pieces. Being very
board bored, he wants to see how many ways he can put \(M\) kings on the board without any of them in position to attack.
Can you help Bob be a little bit less bored and solve the problem for him?
The first and only line contains two integers \(N\) and \(M\) \((1 \le M \le N \le 20)\), the width of the chess board and the number of king pieces.
Output consists of one integer, the number of distinct ways Bob can place the kings without any of them being in range of each other.
Explanation of Output
There are six ways Bob can place the pieces:
1- [K . K . .] 2- [K . . K .] 3- [K . . . K] 4- [. K . K .] 5- [. K . . K] 6- [. . K . K]