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;
using ll = long long;
const int inf = 1e9 + 7;
const ll longinf = 1LL << 60;
const ll mod = 1e9 + 7;
int main() {
  ll a, b;
  cin >> a >> b;
  if (a < b) {
    cout << -1 << endl;
    return 0;
  }
  ll s = a + b;
  ll ng = 3e9, ok = 0;
  while (ng - ok > 1) {
    ll mid = (ok + ng) / 2;
    if (s < 2 * mid * b) {
      ng = mid;
    } else
      ok = mid;
  }
  double ans = 1.0 * s / 2.0 / ok;
  ng = 3e9, ok = 0;
  s = a - b;
  while (ng - ok > 1) {
    ll mid = (ok + ng) / 2;
    if (s < 2 * mid * b) {
      ng = mid;
    } else
      ok = mid;
  }
  ans = min(ans, 1.0 * s / 2.0 / ok);
  cout << fixed << setprecision(12) << ans << endl;
  return 0;
}