#include <bits/stdc++.h>
using namespace std;
double a, b;
int x, cnt;
double dmin(double a, double b) { return (a < b) ? a : b; }
int main() {
  cin >> a >> b;
  if (a < b) {
    cout << -1 << endl;
    return 0;
  }
  if (a == b) {
    cout << a << endl;
    return 0;
  }
  int l = 1;
  int r = 2e9;
  double ans;
  while (l <= r) {
    int mid = (l + r) / 2;
    double x = (a + b) / (2.0 * mid);
    if (x >= b) {
      l = mid + 1;
    } else {
      r = mid - 1;
    }
  }
  if ((a + b) / (2.0 * (l - 1)) >= b)
    ans = (a + b) / (2.0 * (l - 1));
  else
    ans = (a + b) / (2.0 * l);
  l = 1;
  r = 2e9;
  while (l <= r) {
    int mid = (l + r) / 2;
    double x = (a - b) / (2.0 * mid);
    if (x >= b) {
      l = mid + 1;
    } else {
      r = mid - 1;
    }
  }
  if ((a + b) / (2.0 * (l - 1)) >= b)
    ans = dmin((a + b) / (2.0 * (l - 1)), ans);
  else
    ans = dmin((a - b) / (2.0 * l), ans);
  cout << setiosflags(ios::fixed) << setprecision(9) << ans << endl;
  return 0;
}
