#include <bits/stdc++.h>
using namespace std;
double a, b;
bool ck(double x) {
  double ta = a / x;
  int t = round(ta);
  double k = x * t;
  k = (a - k);
  if (t & 1) {
    if (fabs((k - (x - b))) <= 1e-9)
      return true;
    else
      return false;
  }
  if (fabs((k - (b))) <= 1e-9)
    return true;
  else
    return false;
}
int main() {
  int i, j, k;
  double l, r, mid;
  cin >> a >> b;
  if (b > a) {
    cout << -1 << endl;
    return 0;
  }
  if (ck(b) == true) {
    printf("%.9lf\n", b);
    return 0;
  }
  int nl = 0, nr = 1000000000;
  double x1;
  while (nl != nr) {
    int midl = (nl + nr + 1) / 2;
    x1 = (double)(a + b) / (double)(2 * midl + 2);
    if (x1 >= b) {
      nl = midl;
    } else {
      nr = midl - 1;
    }
  }
  double mini = (double)(a + b) / (double)(2 * nl + 2);
  printf("%.9lf\n", mini);
  return 0;
}
