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>
#include <bits/stdc++.h>
using namespace std;
using namespace std;
void solve() {
  int a, b;
  cin >> a >> b;
  if (a < b) {
    cout << -1 << endl;
    return;
  }
  double x1, x2;
  x1 = (a - b) / 2.0;
  x2 = (a + b) / 2.0;
  double res = INT_MAX + 1.0;
  double k;
  k = floor(x1 / b);
  res = min(res, x1 / k);
  k = floor(x2 / b);
  res = min(res, x2 / k);
  cout << fixed << setprecision(9) << res << endl;
}
int main() {
  solve();
  return 0;
}
