a, b = map(int, input().split())
L = []

import math
g = math.gcd(a, b)
if g != 1:
    print(-1)
    exit()

l = (a*b)//g

for x in range(b+1):
    for y in range(a+1):
        z = a*x+b*y
        if z <= l:
            L.append(z)

L = set(L)
print(l-len(L)+1)