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>
namespace SS {}
using ll = long long;
using namespace std;
using namespace SS;
bool prime(int x) {
  for (int i = 2; i <= sqrt(x) + 1; i++) {
    if (i >= x) continue;
    if (x % i == 0) return false;
  }
  return true;
}
struct St {
  int k, a, b;
  friend bool operator<(St a, St b) { return a.k < b.k; }
};
int main() {
  ios::sync_with_stdio(false);
  int a, b;
  cin >> a >> b;
  if (b > a)
    cout << -1;
  else {
    double ans = (a + b) / (double)(2 * ((a + b) / (2 * b)));
    cout << fixed << setprecision(11) << ans;
  }
  return 0;
}