#include<iostream>
using namespace std;
 double b,sum,d=20,a[100001];
 long long n,i;
int main()
{
    cin>>n;
    for (i=1;i<=n;i++)
    {
       cin>>a[i]; 
    }  
    sort(a+1,a+n+1);
    if (n%2==0)
    {
            while (1==1)
           {
           for (i=1;i<=n/2;i++)
           {
               a[i]=a[i]+a[n-i+1];
               sum=sum+(a[i]/d);
           }
           if (n==1){break;}
           else{n=n/2; sort(a+1,a+n+1);}
       }
    }
    else
    {
            while (1==1)
           {
           for (i=1;i<=n/2;i++)
           {
               a[i]=a[i]+a[n-i+1];
               sum=sum+(a[i]/d);
           }
           if (n==1){sum=sum+((a[1]+a[2])/d); break;}
           else{n=n/2; sort(a+1,a+n+2);}
       }
    }
    cout<<sum<<endl;
    system("pause");
    return 0;
}
