var a:array[1..100,1..100] of longint;
  i,j,b,s:longint;
begin
read(b);
for i:=1 to b do
for j:=1 to b do
read(a[i,j]);
for i:=1 to b do
for j:=1 to b do
if (i=j)or(i+j=b+1) then s:=s+a[i,j];
write(s);
end.
/**************************************************************
	Problem: 1726
	User: admin
	Language: Pascal
	Result: Wrong Answer
****************************************************************/