somme des chiffres sur la diagonale matrice

const n = parseInt(readline());
let mat = []
let len = 0
for (let i = 0; i < n; i++) {
    var inputs = readline().split(' ');
    let line = []
    for (let j = 0; j < n; j++) {
        const val = parseInt(inputs[j]);
        line.push(val)
    }
    len = line.length
    mat.push(line)
}
let sum = 0
for(let i = 0; i < len; i++)
    sum += mat[i][len-i-1] + mat[i][i]
let mid = Math.floor(len/2)
if(len%2 != 0)
    sum -= mat[mid][mid]
print(sum)