from math import gcd
a, b = map(int, input().split())
if gcd(a, b) != 1:
    print(-1)
    exit()

c = [1] * (a * b * 4)
for i in range(b * 4):
    for j in range(a * 4):
        v = i * a + j * b
        if v >= a * b * 4:
            break
        c[v] = 0

print(sum(c))