10
-405 -230 252 -393 -390 -259 97 163 81 -129
 3
10000 -10000 10000
 1
-10000
 20
-16 -23 29 44 -40 -50 -41 34 -38 30 -12 28 -44 -49 15 50 -28 38 -2 0

702.333333333333712
 10000.000000000016371
 0.000000000000000
 113.875000000000043

#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:256000000")
using namespace std;
const int INF = 100000000;
const int mod = 1000 * 1000 * 1000 + 7;
const double PI = 3.1415926535897932;
double sqr(double first) { return first * first; };
long double sqr(long double first) { return first * first; };
long long sqr(long long first) { return first * first; };
long long sqr(int first) { return first * 1LL * first; };
long long bpm(long long a, long long n = -2, long long m = mod) {
  n = n >= 0 ? n : m + n;
  long long r = 1;
  while (n) {
    if (n & 1) r = (r * a) % m;
    a = (a * a) % m;
    n >>= 1;
  }
  return r;
}
const int N = 2000002;
int main(int argc, char *argv[]) {
  long double first, second;
  cin >> first >> second;
  cout.precision(10);
  cout.setf(cout.fixed);
  if (second > first) {
    cout << -1 << endl;
  } else if (second == first) {
    cout << second << endl;
  } else {
    long long c = second + first;
    if (c < second) {
      cout << -1 << endl;
      return 0;
    }
    if (c < second * 2) {
      cout << c << endl;
      return 0;
    }
    long long l = 2, r = c;
    while (l != r) {
      long long m = (l + r) >> 1;
      if (c >= second * 2 * m)
        l = m + 1;
      else
        r = m;
    }
    long double cc = c;
    cc = c / 2.0 / (l - 1);
    cout << cc << endl;
  }
  return 0;
}
