// mca.cpp : Defines the entry point for the console application.
//
#include <iostream>
using namespace std; 

int t1, k1, t2, k2;
int main()
{
    cin >> t1 >> k1 >> t2 >> k2;
    cout.precision(6);
    cout << fixed << (double)(t1*t2) / (double)((k1*t2) + (k2*t1));
}