#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("date.in");
ofstream fout("date.out");
int t1, t2, k1, k2;
float rez;

int main()
{
    cin >> t1 >> k1 >> t2 >> k2;
    rez = (double)(t1 * t2) / (t2 * k1 + t1 * k2);
    cout << setprecision(7) << rez;
    return 0;
}