P64789: C401A

#include <iostream>
#include <vector>
using namespace std;


typedef vector< vector<int> > Matriu;


int suma_diagonals(const Matriu& mat) {
    int tam = mat.size();
    int suma = 0;
    int i = 0, j = 0;
    while (i < tam && j < tam) {
        suma += mat[i][j];
        ++i;
        ++j;
    }
    i = tam-1;
    j = 0;
    while (i >= 0 && j < tam) {
        suma += mat[i][j];
        --i;
        ++j;
    }
    if (tam%2 != 0) suma -= mat[tam/2][tam/2];
    return suma;
}

void read_matrix(Matriu& mat){
    int tam = mat.size();
    for (int i = 0; i < tam; ++i) {
        for (int j = 0; j < tam; ++j) {
            cin >> mat[i][j];
        }
    }
}

int main() {
    int tam;
    while (cin >> tam) {
        Matriu mat(tam, vector<int>(tam));
        read_matrix(mat);
        cout << suma_diagonals(mat) << endl;
    }
}

//JosepRivaille