#include<bits/stdc++.h>
using lint=long long;
int main(){
    std::cin.tie(nullptr);std::ios_base::sync_with_stdio(false);
    std::cout.setf(std::ios_base::fixed);std::cout.precision(15);
    double x0,y0,x1,y1;std::cin>>x0>>y0>>x1>>y1;
    auto cal=[=](double y){
        return std::hypot(x0,y-y0)+std::hypot(x1,y-y1);
    };
    double l=-1e3,r=1e3;
    for(lint i=0;i<200;i++){
        double c0=(l+l+r)/3;
        double c1=(l+r+r)/3;
        if(cal(c0)<cal(c1))r=c1;
        else l=c0;
    }
    std::cout<<l<<'\n';
}