#include<iostream>
#include<iomanip>
#include<vector>
#include<string>

using namespace std;

int main(){
  int x1,x2,y1,y2;
  double y;
  cin>>x1>>y1>>x2>>y2;
  y=(double)(x1*y2+x2*y1)/(x1+x2);
  cout<<fixed<<setprecision(6)<<y<<endl;
  return 0;
}