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;
using pii = pair<int, int>;
template <typename... T>
void rd(T&... args) {
  ((cin >> args), ...);
}
template <typename... T>
void wr(T... args) {
  ((cout << args << " "), ...);
  cout << endl;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  int a, b;
  cin >> a >> b;
  if (b > a) return cout << -1, 0;
  printf("%.9lf", double(a + b) / (2.0 * ((a + b) / (2 * b))));
  return 0;
}