#include <bits/stdc++.h>
using namespace std;
long double ans = 1e10;
void check(int64_t x, int64_t y) {
  if (x < 0) return;
  long double k = 1. * x / 2;
  int64_t l = 1, r = 2e9 + 1;
  while (r - l > 1) {
    int64_t m = (l + r) / 2;
    if (k / m >= y)
      l = m;
    else
      r = m;
  }
  if (k / l >= y) ans = min(ans, k / l);
}
signed main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int64_t a, b;
  cin >> a >> b;
  if (b > 0) {
    check(a + b, b);
    check(a - b, b);
    if (ans == 1e10)
      cout << fixed << setprecision(12) << -1 << "\n";
    else
      cout << fixed << setprecision(12) << ans << "\n";
  }
  return 0;
}
