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?

Dreamoon 想爬上一个有 $n$ 级台阶的楼梯。他每次可以爬 $1$ 级或 $2$ 级台阶。Dreamoon 希望总步数是某个整数 $m$ 的倍数。

满足该条件的前提下，他爬上楼梯顶部所需的最少步数是多少？

The single line contains two space separated integers n, m (0 < n ≤ 10000, 1 < m ≤ 10).

单行包含两个以空格分隔的整数 $n$、$m$（$0 < n \leq 10000$，$1 < m \leq 10$）。

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.

输出一个整数——满足条件的最小移动次数，且该次数是 $m$ 的倍数。如果不存在满足条件的爬升方式，则输出 $-1$。

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.

对于第一个样例，Dreamoon 可以通过以下步长序列在 6 步内爬升：{2, 2, 2, 2, 1, 1}。

对于第二个样例，仅有三个有效的步长序列：{2, 1}、{1, 2} 和 {1, 1, 1}，对应步数分别为 2、2 和 3。这些数均不是 5 的倍数。