#include<stdio.h>
using namespace std;
int t1, k1, t2, k2;
double T1, T2, K1, K2;

int main()
{
 //   freopen("a.in","r",stdin);
  //  freopen("a.out","w",stdout);
    scanf("%d %d %d %d",&t1,&k1,&t2,&k2);
    T1=t1;  K1=k1;  T2=t2;  K2=k2;
    printf("%.5f",1.0/(K1/T1+K2/T2));
    return 0;
}