A, B = [int(x) for x in input().split()]

A, B = min(A, B), max(A, B)

if A == 1 or B == 1:
    print(0)
    exit()


for i in range(2, 101):
    if A % i == 0 and B % i == 0:
        print(-1)
        exit()


x = set()

j = 1
while len(x) != B:
    x.add((j * A) % B)
    j += 1

ans = 0
for i in range(j * A):
    if (i % A) % B == 0:
        pass
    elif i % A == 0:
        pass
    elif i % B == 0:
        pass
    elif (i % B) % A == 0:
        pass
    else:
        ans += 1

print(ans // 2)