#include <cstdio>

using namespace std;
int t1,t2,k1,k2,a,b;
double c;
int main()
{
    scanf ("%d %d %d %d", &t1, &k1, &t2, &k2);
    a=t1*t2;
    b=t2*k1+t1*k2;
    c=1.0*a/b;
    printf ("%lf", c);
    return 0;
}