#include<bits/stdc++.h>
using namespace std;
double k;
double a,b,c,d,s,p;
double f(double x)
{
	return -b/k+a;
}
int main()
{
	
    cin>>a>>b>>c>>d;
    b=-b;
    k=(b-d)/(a-c);
    s=f(0);
    printf("%.10lf",s);
	return 0;
	
}
#include<bits/stdc++.h>
#include<iostream>
#include<string>
#include<vector>
#include<iomanip>
using namespace std;
int main()
{
    double x1,y1,x2,y2;
    cin >> x1 >> y1 >> x2 >> y2;
    double t = (((x1 * y2) + (x2 * y1))/(y1 + y2));
    cout <<fixed << setprecision(10) <<t << endl;
}
