#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 5;
const int M = 2e5 + 5;
const int INF = 0x3f3f3f3f;
const int MOD = 1e9 + 7;
const double eps = 1e-6;
int a, b;
int dcmp(double x) {
  if (fabs(x) < eps) return 0;
  if (x > eps) return 1;
  return -1;
}
int main() {
  cin >> a >> b;
  if (!a && !b) return 0 * puts("0");
  double best = 1e18;
  if (a > b) {
    double v = (a - b + 0.0) / 2;
    int up = v / b, down = 1;
    if (up >= down) best = min(best, v / up);
  }
  double v = (a + b + 0.0) / 2;
  int up = (v / b), down = 1;
  if (up >= down) best = min(best, v / up);
  if (fabs(best - 1e18) < eps)
    puts("-1");
  else
    printf("%.12f", best);
  return 0;
}
