Problems
New-year tree
New-year tree
For decoration of the New-year tree Peter has in the order a garland from the n lamps and k of different paints for their painting. How many methods can he to do it, if he must no two identical colors be alongside?

Input data
The amount of lamps is n, the amount of different paints is k (1 ≤ k, n ≤ 15).
Output data
Print the number of painting ways. If Peter can not paint a garland after the described requirements, to show out -1.
Examples
Input example #1
6 2
Output example #1
2