#include <bits/stdc++.h>
using namespace std;
void panic() {
  cout << -1 << endl;
  exit(0);
}
const double EPS = 1e-9;
unsigned long long x, y;
double f(double l, double x) {
  while (x > l * 2 + EPS) {
    x -= l * 2;
  }
  return l - x;
}
int main() {
  ios::sync_with_stdio(false);
  cout.precision(10);
  cin >> x >> y;
  if (y > x) panic();
  double coef = double(x) / y;
  unsigned long long v = x - x % y;
  if (v % (2 * y) == 0) v -= y;
  double left = 0, right = y;
  double value;
  while (left < right + EPS) {
    double mid = (left + right) / 2;
    double curcoef = (v + mid) / (y - mid);
    if (abs(curcoef - coef) < EPS) {
      value = v + mid;
      break;
    } else if (curcoef > coef) {
      right = mid;
    } else {
      left = mid;
    }
  }
  cout << fixed << y * x / value << endl;
}
