#include "bits/stdc++.h"
#define _CRT_SECURE_NO_WARNINGS
#define rep(i,n) for(int i = 0;i < n;i++)
#define REP(i,n,k) for(int i = n;i < k;i++)
#define P(p) cout << (p) << endl;
#define sP(p) cout << setprecision(15) << fixed << p << endl;
#define vi vector<int>
#define mp(a,b) make_pair(a,b)
using namespace std;typedef long long ll;typedef unsigned long long ull;int dx[] = { 1, -1 , 0 , 0};int dy[] = { 0,  0,  1, -1};

void solve() {
  double vl,vr,d,w;
  cin >> vl >> vr >> d >> w;
  double ans = w * (d/(vr+vl));
  sP(ans);
}

int main() {
	solve();
	return 0;
}