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?

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.

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.