CF476A Dreamoon and Stairs

Dreamoon wants to climb up a stair of $ n $ steps. He can climb $ 1 $ or $ 2 $ steps at each move. Dreamoon wants the number of moves to be a multiple of an integer $ m $ . What is the minimal number of moves making him climb to the top of the stairs that satisfies his condition?

The single line contains two space separated integers $ n $ , $ m $ ( $ 0

Print a single integer — the minimal number of moves being a multiple of $ m $ . If there is no way he can climb satisfying condition print $ -1 $ instead.

For the first sample, Dreamoon could climb in 6 moves with following sequence of steps: {2, 2, 2, 2, 1, 1}. For the second sample, there are only three valid sequence of steps {2, 1}, {1, 2}, {1, 1, 1} with 2, 2, and 3 steps respectively. All these numbers are not multiples of 5.