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

if (A%2)+(B%2) == 0:
    print(-1)
else:
    dp = [1]*(10**6)
    dp[0] = 0
    for i in range(10**6):
        if i >= A:
            dp[i] = dp[i-A]
        if i >= B:
            dp[i] = min(dp[i-B],dp[i])

    print(sum(dp))