Problem: 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.

Input

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

Output

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.

Examples

Input

3 1


Output

1.000000000000


Input

1 3


Output

-1


Input

4 1


Output

1.250000000000

Note

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

<image> <image>

Solution (in CPP):

Solution:
#include <bits/stdc++.h>
using namespace std;
double x, y;
int main() {
  cin >> x >> y;
  if (y > x) {
    cout << -1;
    return 0;
  }
  double b = x + y;
  double base = 1;
  int divnum1 = b * 0.5 / y;
  double x1 = b / (1.0 * divnum1) * 0.5;
  double a = fabs(y - x);
  int divnum2 = a * 0 / 5 / y;
  double x2 = a / (1.0 * divnum2) * 0.5;
  cout << fixed << setprecision(12) << min(x1, x2) << endl;
}