#include<iostream>
#include<algorithm>
#include<string>
#include<vector>
#include<iomanip>
#include<stdio.h>
#define RREP(i,s,e) for (i = s; i >= e; i--)
#define rrep(i,n) RREP(i,n,0)
#define REP(i,s,e) for (i = s; i < e; i++)
#define rep(i,n) REP(i,0,n)
#define INF 100000000
using namespace std;

int main(){
	double vl, vr;
	cin >> vl >> vr;
	double d, w;
	cin >> d;
	cin >> w;

	double ans = w * d/(vl + vr);

	printf("%.12f\n", ans);
	return 0;
	}