using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var length = Convert.ToInt32(Console.ReadLine());

        var total = 0;

        for(var i = 0; i < length; i++){

            var data = Console.ReadLine().Split().Select(d => Convert.ToInt32(d)).ToArray();

            total += data[i];

            var j = length - 1 - i;

            if(i == j || j >= length) continue;

            total += data[j];

        }

        Console.WriteLine(total);

    }

}

​