#include <bits/stdc++.h>
using namespace std;
long long a, b, r, e, k;
double res1, res2, res;
bool co1, co2;
int main() {
  ios::sync_with_stdio(0);
  scanf("%d %d", &a, &b);
  if (a == b) {
    double p = double(a);
    printf("%0.9lf", p);
    return 0;
  }
  res1 = res2 = 5000000000;
  r = a - b;
  e = a + b;
  co1 = co2 = true;
  if (e / b >= 2) {
    k = e / b;
    if (k % 2) k--;
    res1 = e / double(k);
  } else
    co1 = false;
  if (r > 0) {
    e = r;
    if (e / b >= 2) {
      k = e / b;
      if (k % 2) k--;
      res2 = e / double(k);
    } else
      co2 = false;
  } else
    co2 = false;
  if (!co1 && !co2)
    printf("-1");
  else {
    res = min(res1, res2);
    printf("%0.9f", res);
  }
  return 0;
}
