#include <bits/stdc++.h>
using namespace std;
const double eps=1e-8;

double Xa,Xb,Ya,Yb;
inline double calc(double yy){
    double t=Yb-yy;
    double l=yy-Ya;
    return sqrt(Xb*Xb+t*t)+sqrt(Xa*Xa+l*l);
}
int main() {
    cout<<setprecision(10);
    cout<<fixed;
    cin>>Xa>>Ya>>Xb>>Yb;
    if(Ya>Yb){
        swap(Xa,Xb);
        swap(Ya,Yb);
    }
    {
        double top=Yb;
        double bottom=Ya;
        while(top-bottom>eps){
            double m1=(top*2+bottom)/3;
            double m2=(top+bottom*2)/3;
            if(calc(m1)<calc(m2))bottom=m2;
            else top=m1;
        }
        cout<<top<<endl;
    }
    return 0;
}