CF578A.A Problem about Polyline

There is a polyline going through points (0, 0) – (x, x) – (2_x_, 0) – (3_x_, x) – (4_x_, 0) – ... - (2_kx_, 0) – (2_kx_ + x, x) – ....

We know that the polyline passes through the point (a, b). Find minimum positive value x such that it is true or determine that there is no such x.

存在一条折线，依次经过点 $(0, 0) – (x, x) – (2x, 0) – (3x, x) – (4x, 0) – \ldots – (2kx, 0) – (2kx + x, x) – \ldots$。

已知该折线经过点 $(a, b)$。求满足条件的最小正实数 $x$；若不存在这样的 $x$，则判定为无解。

Only one line containing two positive integers a and b (1 ≤ a, b ≤ 109).

仅一行，包含两个正整数 $a$ 和 $b$（$1 ≤ a, b ≤ 10^9$）。

Output the only line containing the answer. Your answer will be considered correct if its relative or absolute error doesn't exceed 10 - 9. If there is no such x then output  - 1 as the answer.

输出唯一一行，包含答案。只要你的答案的相对误差或绝对误差不超过 $10^{-9}$，即视为正确。如果不存在满足条件的 $x$，则输出 $-1$ 作为答案。

You can see following graphs for sample 1 and sample 3.

您可以参见样例 1 和样例 3 对应的以下图表。