from math import gcd

a, b = map(int, input().split())
if gcd(a, b) > 1:
    print(-1)
else:
    seen = set()
    for i in range(0, a * b + 1, a):
        for j in range(0, a * b + 1, b):
            if i + j > a * b:
                break
            seen.add(i + j)
    ans = set(range(a * b + 1)) - seen
    print(len(ans))