#include<iostream>
#include<cmath>

using namespace std;

int main(){
    int x, y, k;
    cin >> x >> y;
    if(x < y)
        return cout << "-1", 0;
    k = (y+x)/y;
    if(k%2 == 1)
        k--;
    cout.precision(18);
    cout.fixed;
    cout << double(x+y)/double(k);
}