import math

def chicken_mcnugget(m, n):
    return m * n - m - n

A, B = map(int, input().split())
if math.gcd(A, B) != 1:
    print(-1)
else:
    lim = chicken_mcnugget(A, B)
    sieve = [0] * (lim+1)
    for a in range(0, lim // A + 1):
        for b in range(0, (lim - A * a) // B + 1):
            sieve[A*a + B*b] = 1
    ans = sum(x != 0 for x in sieve)
    print(ans)