#include<cstdio>
#include<algorithm>
using namespace std;
int main()
{
    double f=0;
    int i,n,a[10000],d,p;
    scanf("%d",&n);
    for(i=0;i<n;i++)
    {
        scanf("%d",&a[i]);
    }
    sort(a,a+n);
    while(n!=1)
    {
        p=0;
        for(i=0;i<n;i+=2)
        {
            f+=double(double(a[i]+a[i+1])/double(20));
            //printf("%.2fl %d %d\n",f,a[i],a[i+1]);
            a[p++]=a[i]+a[i+1];
        }
        n/=2;
    }
    /*d=f*100;
    if(d%10>4)d+=10;
    printf("%d ",d);
    d-=d%10;
    printf("%d ",d);
    f=double(double(d)/double(100));*/
    printf("%.2f\n",f);
    return 0;
}
