A,B = map(int,input().split())

dp = [0] * (10**6)
dp[0] = 1

for i in range(len(dp)):

    if i+A < len(dp):
        dp[i+A] |= dp[i]
    if i+B < len(dp):
        dp[i+B] |= dp[i]


ans = len(dp) - sum(dp)

if ans >= 10**5:
    print (-1)
else:
    print (ans)