# -*- coding: utf-8 -*-

a,b = map(int,input().split())

ans = 0
for c in range(1,a+b+1):
    if a != b and b != c and c != a:
        if (b + c) % a == 0 and (a + c) % b == 0 and (b + a) % c == 0:
            ans = c
            break
else:
    ans = -1

print(ans)