import math

a, b = map(int, input().split())

if math.gcd(a, b) != 1:
    print(-1)
else:
    s = [False for _ in range(2*a*b+1)]
    for i in range(b):
        for j in range(a):
            s[a*i+b*j] = True

    print(s[:a*b].count(False))