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;
inline void fast() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
}
const long long int mod = 1e9 + 7;
long long int pmod(long long int x, long long int n) {
  long long int r = 1;
  while (n) {
    if (n & 1LL) r = (r * x) % mod;
    x = (x * x) % mod;
    n = n / 2;
  }
  return r;
}
void solve() {
  long long int a, b;
  cin >> a >> b;
  if (a < b) {
    cout << -1;
    return;
  }
  long long int d1 = (a + b) / 2;
  d1 /= b;
  d1 = d1 * 2;
  double ans = (double)(a + b) / d1;
  cout << setprecision(10) << ans;
}
int main() {
  fast();
  solve();
}