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A. Dreamoon and Stairs

time limit per test

1 secondmemory limit per test

256 megabytesinput

standard inputoutput

standard outputDreamoon 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.

Examples

Input

10 2

Output

6

Input

3 5

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.

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