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 ans, n, m;
inline double min(double x, double y) { return x < y ? x : y; }
int main() {
  cin >> n >> m;
  ans = 1000000000;
  if (n < m) {
    puts("-1");
    return 0;
  }
  if (!m) {
    puts("0");
    return 0;
  }
  double k = n - m;
  int v = floor((k / 2) / m);
  ans = min(ans, k / 2 / v);
  k = n + m;
  v = floor((k / 2) / m);
  ans = min(ans, k / 2 / v);
  printf("%.9lf\n", ans);
  return 0;
}