Dreamoon and Stairs

Description:

Input:

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

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.

Sample Input:

10 2

Sample Output:

6

Sample Input:

3 5

Sample Output:

-1

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.