#include <iostream>
#include <iomanip>
using namespace std;
int main(){
  long long vl,vr,d,w;cin>>vl>>vr>>d>>w;
  long double A = (long double)2*w*w/((w+vr)*(w+vl));
  long double B = (long double)((w-vl)*(w-vr))/((w+vr)*(w+vl));
  cout << fixed << setprecision(10) << (double)d*A/(1-B) << endl;
}