## Placing Kings

Points: 10 (partial)
Time limit: 1.0s
Memory limit: 256M

Author:
Problem type

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]