#include <cstdio>
#include <algorithm>

using namespace std;

int main ()
{
    double t11, k11, t22, k22;
    scanf ("%lf %lf %lf %lf", &t11, &k11, &t22, &k22);

    printf ("%.5f", t11 * t22 / (t22 * k11 + t11 * k22));
    return 0;
}