#include <bits/stdc++.h>
#define p(s) cout<<(s)<<endl
using namespace std;

int main(){
	int xa,ya,xb,yb;
	cin>>xa>>ya>>xb>>yb;
	double ans=(yb-(double)(yb-ya)/(xa+xb)*xb);
	printf("%.7lf",ans);
	return 0;
}