#include <iostream>
#include <iomanip>
using namespace std;

int t1, k1, t2, k2, total, v1, v2;
double raspuns;

int main()
{
    cin >> t1 >> k1 >> t2 >> k2;
    total = t1*t2;
    v1 = t2;
    v2 = t1;
    raspuns = double(total)/double(v1*k1 + v2*k2);
    cout << setprecision(7) << fixed << raspuns;
}