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

double t1, k1, t2, k2;

int main(){

scanf("%lf %lf %lf %lf", &t1, &k1, &t2, &k2);
printf("%.5lf", t1 / k1 + t2 / k2);

return 0;
}