# 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?

## 1 Input

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

## 2 Output

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.

## 3 Examples

### 3.1 input1

``````10 2
``````

### 3.2 output1

``````6
``````

### 3.3 input2

``````3 5
``````

### 3.4 output2

``````-1
``````

### 3.5 Note

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.

 `````` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 `````` ``````#include using namespace std; int main(){ int x,n,m; cin>>n>>m; if(n

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