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 .

输入描述:

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

输出描述:

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.

备注:

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