#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstdlib>
#include<vector>
#include<map>
#include<queue>
#include<string>
#include<sstream>
#include<cmath>
#include<numeric>
using namespace std;

int main(){

  double vl, vr, d, w;
  cin >> vl >> vr >> d >> w;

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

  printf("%.10f\n", ans);

  return 0;
}