#include <bits/stdc++.h>
using namespace std;
double Min(double x, double y) { return x < y ? x : y; }
int main() {
  double a, b;
  double aab, asb, k1, k2;
  while (cin >> a >> b) {
    aab = a + b;
    asb = a - b;
    if (asb == 0) {
      printf("%.12lf\n", b);
      continue;
    }
    if (asb < 0) {
      printf("-1\n");
      continue;
    }
    k1 = floor(aab / 2.0 / b);
    k2 = floor(asb / 2.0 / b);
    printf("%.12lf\n", Min(aab / 2.0 / k1, asb / 2.0 / k2));
  }
  return 0;
}
