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;
const double INF = 1e18;
double solve(long long x, long long y) {
  long long k = x / (2 * y);
  if (k == 0) return INF;
  return (double)x / (double)(2 * k);
}
int main() {
  long long x, y;
  cin >> x >> y;
  if (x < y) {
    printf("-1\n");
    return 0;
  }
  if (x == y) {
    printf("%d\n", x);
    return 0;
  }
  printf("%.12lf\n", min(solve(x - y, y), solve(x + y, y)));
  return 0;
}