#include <bits/stdc++.h>
using namespace std;
const double pi = acos(-1.0);
const int inf = 0x3f3f3f3f;
const double eps = 1e-10;
const long long INF = (1LL << 60);
int main() {
  int a, b;
  scanf("%d%d", &a, &b);
  if (a == b) {
    printf("%d\n", a);
    return 0;
  }
  double ans = -1;
  if (a >= b) {
    int k = (int)floor((a * 1.0 / b - 1) / 2);
    ans = (a - b) * 1.0 / (2 * k);
    ans = min(ans, (a + b) * 1.0 / (2 * k + 2));
  }
  printf("%0.12f\n", ans);
  return 0;
}
