#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:1024000000,1024000000")
using namespace std;
int main() {
  double a, b;
  cin >> a >> b;
  long long l = 0, r = 1e10;
  double ans = 1e20;
  while (l <= r) {
    long long mid = (l + r) / 2;
    double x = (a + b) / 2 / (mid + 1);
    if (x + 1e-10 >= b) {
      l = mid + 1;
      ans = min(ans, x);
    } else
      r = mid - 1;
  }
  l = 1, r = 1e10;
  while (l <= r) {
    long long mid = (l + r) / 2;
    double x = (a - b) / 2 / mid;
    if (x + 1e-10 >= b) {
      l = mid + 1;
      ans = min(ans, x);
    } else
      r = mid - 1;
  }
  if (ans == 1e20)
    cout << -1 << endl;
  else
    printf("%.10f\n", ans);
  return 0;
}
