#include <iostream>
#include <string>
#include <vector>
#include <cmath>
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <cstdio>
#include <functional>
#include <set>
#include <sstream>
#include <map>
#include <queue>
#include <stack>

using namespace std;

int main(){


    double a,b,c,d;
    cin>>a>>b>>c>>d;
    
    if(a==c) cout<<(a*d+b*c)/(a+c)<<endl;
    else {
        double p1=(a*d-b*c)/(a-c);
        double p2=(a*d+b*c)/(a+c);
        
        if(p1<0.0) cout<<p2<<endl;
        else{
            double d1=a*a+(b-p1)*(b-p1)+c*c+(d-p1)*(d-p1);
            double d2=a*a+(b-p2)*(b-p2)+c*c+(d-p2)*(d-p2);
            if(d1<=d2) cout<<p1<<endl;
            else cout<<p2<<endl;           
        }
    }
    return 0;

}