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>
using namespace std;
const int N = 2e6 + 7;
const int INF = 1e18 + 7;
const double EPS = 1e-12;
const int P = 37;
const long long MOD = 1e9 + 7;
int fx[] = {1, -1, 0, 0, 1, 1, -1, -1};
int fy[] = {0, 0, 1, -1, -1, 1, -1, 1};
double x, y, xx, yy;
int main() {
  cin >> x >> y;
  if (y > x) {
    cout << -1;
    return 0;
  }
  xx = x + y;
  yy = xx / 2;
  while (yy / 2 >= y) {
    xx /= 2;
    yy /= 2;
  }
  printf("%.10lf", yy);
  return 0;
}
