from math import *
x = int(input("Введите размер матрицы\n"))
matrix = [0] * x
for i in range(x):
    matrix[i] = [0] * x

sum = 0

for i in range(x):
    matrix[i] = list(map(int, input().split()))
    if (len(matrix[i]) != 3):
        exit(1)

for i in range(x):
    for j in range(x):
        if (i == j):
            sum += matrix[i][j]



print(sum)