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

int main() {
  double sx, sy, gx, gy; 
  cin >> sx >> sy >> gx >> gy;
  double ans;
  ans = ((sx * gy) + (gx * sy)) / (sy + gy);
  cout <<fixed<<setprecision(6)<< ans <<endl;
}
#include<bits/stdc++.h>
using namespace std;
int main(){
    int a,b,c,d;
    cin >> a >> b >> c >> d;
    cout << b - c << endl;
    return 0;
}