#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);
    long double x0,y0,x1,y1;std::cin>>x0>>y0>>x1>>y1;
    auto cal=[=](long double y){
        return std::sqrt(x0*x0+(y-y0)*(y-y0))+std::sqrt(x1*x1+(y-y1)*(y-y1));
    };
    long double l=-1e3,r=1e3;
    for(lint i=0;i<150;i++){
        long double c0=(l+l+r)/3;
        long double c1=(l+r+r)/3;
        if(cal(c0)<cal(c1))r=c1;
        else l=c0;
    }
    std::cout<<l<<'\n';
}