## Placing Kings

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?

#### Input Specification

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 Specification

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.

#### Sample Input

`5 2`

#### Sample Output

`6`

#### 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]
```

## Comments

Bruce force