#include<cstdio>
#include<algorithm>
using namespace std;
int cmp(int a,int b)
{
    if(a>b)return 1;
    return 0;
}
int main()
{
    double f=0;
    int i,n,a[100000],d,p,k,v;
    scanf("%d",&n);
    for(i=0;i<n;i++)
    {
        scanf("%d",&a[i]);
    }
    make_heap(a,a+n,cmp);
    while(n!=1)
    {
        p=0;
        k=a[0];
        pop_heap(a,a+(n),cmp);
        /*for(i=0;i<n;i++)
        {
            printf("%d ",a[i]);
        }*/
        n--;
        f+=double(double(k+a[0])/double(20));
        //printf("\n %d %d %d %d\n",k,a[0],n);
        k=k+a[0];
        pop_heap(a,a+(n),cmp);
        /*for(i=0;i<n;i++)
        {
            printf("%d ",a[i]);
        }*/
        a[n-1]=k;
        /*for(i=0;i<n;i++)
        {
            printf("%d %d %d\n",a[i],n,k);
        }
        v=n;
        while(v)
        {
            if(v%2==0 && a[v]<a[(v-2)/2])
            {
                a[(v-2)/2]=a[v];
                v=(v-2)/2;
            }
            else if(v%2==1 && a[v]<a[(v-1)/2])
            {
                a[(v-1)/2]=a[v];
                v=(v-1)/2;
            }
            else break;
        }*/
        push_heap(a,a+n,cmp);
    }
    printf("%.2f\n",f);
}
