#include<bits/stdc++.h>
using namespace std;
using Int = long long;

struct Precision{
  Precision(){
    cout<<fixed<<setprecision(12);
  }
}precision_beet;

//INSERT ABOVE HERE
signed main(){
  double a,b,c,d;
  cin>>a>>b>>c>>d;
  cout<<c/(a+b)*d<<endl;
  return 0;
}