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;
int main() {
  int a, b;
  scanf("%d %d", &a, &b);
  if (a < b)
    printf("-1");
  else {
    double ans;
    int xx = (a - b) / (2 * b);
    int yy = (a + b) / (2 * b);
    double xxx = ((a - b) * 1.0) / (2.0);
    double yyy = ((a + b) * 1.0) / (2.0);
    if (xxx < 0.000000001)
      ans = yyy;
    else if (yyy < 0.000000001)
      ans = xx;
    else
      ans = min(xxx / (xx * 1.0), (yyy / (yy * 1.0)));
    printf("%.12lf", ans);
  }
}