def gcd(x, y):
    while y:
        x, y = y, x % y
    return x

A, B = map(int, input().split())

g = gcd(A, B)

if g > 1:
    print(-1)
else:
    dp = [0 for _ in range(A * B + 1)]
    dp[0] = 1
    for i in range(A * B):
        if dp[i]:
            if i + A <= A * B:
                dp[i + A] = 1
            if i + B <= A * B:
                dp[i + B] = 1

    print(dp.count(0))