var88_2009_s2_5

#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
int a[25][25];
int main ()
{
    int  i,j,n,c=0;
    float s=0,m;
    cin>>n;
    for (i=1;i<=n;i++)
    {
        for (j=1;j<=n;j++)
        {
            cin>>a[i][j];
        }
    }
    for (i=1;i<=n;i++)
    {
        for (j=1;j<=n;j++)
        {
            if (i<j)
            {
                if (a[i][j]>0)
                {
                    s=s+a[i][j];
                    c++;
                }

            }
        }
    }
    m=1.00*s/c;
    cout<<setprecision(2)<<fixed<<floor(m*100)/100;
    return 0;
}
/*
-1 2 -4 5
0 6 3 1
2 4 2 0
3 -5 1 -3
*/