#include<cstdio>
using namespace std;
int main()
{
   // freopen("date.in","r",stdin);
   // freopen("date.out","w",stdout);
    int t1,k1,t2,k2;
    double rez;
    scanf("%d%d%d%d",&t1,&k1,&t2,&k2);
    rez=1.000/(k1/t1+k2/t2);
    printf("%.4lf",rez);
    return 0;
}