n = int(input()) a =[ input().split() for x in range(n) ] # a =[] # for x in range(n): # tmp = input().split() #['2','3','4'] # a.append(tmp) res = 0 for x in range(n): for y in range(n): if x==y or x+y ==n-1: res = res + int(a[x][y]) print(res)