#include <bits/stdc++.h>

using namespace std;

int t1, t2, k1, k2;
int main()
{
  //  freopen("input", "r", stdin);
   // freopen("output", "w", stdout);

    cin >> t1 >> k1 >> t2 >> k2;
    cout << (double)t1*t2/(k1*t2 + k2*t1) << '\n';

    return 0;
}