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

if (A%2)+(B%2) == 0:
    print(-1)
else:
    dp = [[0]*(10**5) for i in range(2)]
    dp[0][A] = 1
    dp[0][0] = 1
    for i in range(A+1,10**5):
        if i%A == 0:
            dp[0][i] = 1

    for i in range(10**5):
        if i < B:
            dp[1][i] = dp[0][i]
        else:
            dp[1][i] = max(dp[1][i-B],dp[0][i])

    print(10**5-sum(dp[1]))