Sum the elements on both diagonals of a square matrix. If the matrix has an odd size, the centre element lies on both diagonals — count it only once.
Line 1: an integer n.
The next n lines each contain n integers (the matrix rows).
The sum of the primary diagonal plus the secondary diagonal, counting the centre once.
Example 1
3 1 2 3 4 5 6 7 8 9
25
Primary 1+5+9, secondary 3+5+7, minus the centre 5 → 25.
Example 2
2 1 2 3 4
10
No centre overlap: (1+4) + (2+3) = 10.
Example 3
1 5
5
A single element is the whole of both diagonals — 5.
The primary diagonal is element (i, i); the secondary diagonal is element (i, n-1-i).
When n is odd, the middle element (i == n-1-i) is on both — add it once, not twice.
For each row i add m[i][i] and m[i][n-1-i]; when i == n-1-i (the centre) subtract one copy so it counts once.
import sys
lines = [l for l in sys.stdin.read().split('\n') if l.strip() != '']
n = int(lines[0])
m = [list(map(int, lines[1 + i].split())) for i in range(n)]
s = 0
for i in range(n):
s += m[i][i] + m[i][n - 1 - i]
if i == n - 1 - i:
s -= m[i][i]
print(s)
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