#include <iostream>
#include <stdio.h>
using namespace std;

int mx = -1;


int main()
{
   ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
   double t1, k1, t2, k2, res;
   cin >> t1 >> k1 >> t2 >> k2;
   res = (t1 * t2) / (t2 * k1 + t1 * k2);
   printf("%lf6", res);




   return 0;
}