from sys import stdin


def main():
    input = lambda: stdin.readline()[:-1]
    A, B = map(int, input().split())

    if (A > 0 and B > 0) or (not A and B != 0):
        print(-1)
        return
    elif A == B == 0:
        print(1)
        return

    ans = abs(B // A)
    if ans % 2:
        print(-1)
    else:
        print(max(1, ans))


main()