A Problem about Polyline

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

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

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.

3 1

1.000000000000

1 3

-1

4 1

1.250000000000

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