#include <cstdio>
#include <cctype>
using namespace std;

double t1, k1, t2, k2, T1, T2;

int main(){

scanf("%lf %lf %lf %lf", &t1, &k1, &t2, &k2);

T1 = t1 / k1;
T2 = t2 / k2;

printf("%lf", (T1 * T2) / (T1 + T2));

return 0;
}