1
n, m = map(int, input().split())
2
 
3
ans = -1
4
i = 0
5
while i * 2 <= n:
6
    k = i + (n - i * 2)
7
    if k % m == 0:
8
        if ans == -1 or ans > k:
9
            ans = k
10
    i += 1
11
print(ans)