#include <iostream>
#include <vector>
#include <algorithm>
#include <set>
#include <map>
using namespace std;
typedef long long int ll;

int main(){
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	double x,y,xx,yy;
	cin >> x >> y >> xx >> yy;
	printf("%.9f\n",y-x*(y-yy)/(xx+x));
}