import math
A, B = map(int, input().split())
g = math.gcd(A, B)
if g != 1:
    exit(print(-1))
if A == 1 or B == 1:
    exit(print(0))

LCM = math.lcm(A, B)
DP = [False] * (LCM + 1)
for x in range(0, B + 1):
    for y in range(0, A + 1):
        if A*x + B*y <= LCM:
            DP[A*x + B*y] = True

ans = []
for i in range(1, LCM):
    if not DP[i]:
        ans.append(i)
print(len(ans))