#include <cstdio>
#include <algorithm>
using namespace std;
int n,ch[100000];
double sum,a,b;
int main(){
    scanf("%d",&n);
    for (int i=0;i<n;++i){
        scanf("%d",&ch[i]);
    }
    sort(ch,ch+n);
    for (int i=0;i<n;++i){
        a=ch[i];b=ch[i+1];
        ch[i+1]=ch[i]+ch[i+1];
        sum+=(a+b)/20.0;
    }
    printf("%.2lf\n",sum);
    return 0;
}
