#include <iostream>
#include <iomanip>

using namespace std;
int t1,t2,k1,k2;
int p,s;
double t;

int main()
{
    cin >> t1 >> k1 >> t2 >> k2;
    p = t1 * t2;
    s = t2 * k1 + t1 * k2;
    t = (double)p / (double)s;
    cout << fixed << setprecision(4) <<t;
}