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;
int main() {
  double a, b;
  cout.setf(ios::fixed);
  cout.precision(12);
  cin >> a >> b;
  if (a - b < 0)
    cout << -1 << endl;
  else if (a == b)
    cout << a << endl;
  else {
    double x = -1;
    double c = a + b;
    double t = c / (b);
    t = floor(t);
    if ((int)t % 2 == 1) --t;
    if (t != 0) x = c / t;
    cout << x << endl;
  }
}