#include <cstdio>
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

int main(void){

  double vl,vr,d,w;
  scanf("%lf %lf %lf %lf",&vl,&vr,&d,&w);

  printf("%.10f\n",w*(d/(vl+vr)));
  return 0;
}