using System;

namespace Primary_diagonal
{
    class Program
    {
        static void Main(string[] args)
        {
            int matrixSize = int.Parse(Console.ReadLine());

            var matrix = new int[matrixSize, matrixSize];
            long primaryDiagonalSum = 0;

            for (int row = 0; row < matrixSize; row++)
            {
                var numbers = Console.ReadLine();
                var singleNum = numbers.Split(new char[] {' '}, StringSplitOptions.RemoveEmptyEntries);
                for (int col = 0; col < matrixSize; col++)
                {
                    matrix[row, col] = int.Parse(singleNum[col]);
                }
            }
            for (int row = 0; row < matrix.GetLength(0); row++)
            {
                for (int col = 0; col < matrix.GetLength(1); col++)
                {
                     primaryDiagonalSum += matrix[row, col] + matrix[row + 1, col + 1] + matrix[row + 2, col + 2];
                    Console.WriteLine(primaryDiagonalSum);
                    return;
                }
            }

        }
    }
}