CF476A.Dreamoon and Stairs

普及/提高-

通过率：0%

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题目描述

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?

输入格式

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

输出格式

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.

输入输出样例

输入#1
```
10 2
```
输出#1
```
6
```
输入#2
```
3 5
```
输出#2
```
-1
```

说明/提示

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.

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