n = int(input()) # a=[ [ int(t) for t in input().split()] for x in range(rows) ] #二维列表生成式 a = [ list(map(int, input().split())) for x in range(n)] #1维列表生成式 + map函数的使用 res=0 for x in range(n): for y in range(n): if x==y or x+y == n-1: res+=a[x][y] print(res)