如何正确编写，很菜求解答

练习遇到的，实在不会，求解答，个人对c++感兴趣，初学状态
感谢

#include
using namespace std;

int main()
{
constexpr int ROW = 5;
constexpr int COL = 5;

int a[ROW][COL];
int sum = 0;

for (int i = 0; i < ROW; ++i) {
    for (int j = 0; j < COL; ++j) {
        cin >> a[i][j];
        if (i == j) {
            sum += a[i][j];
        }
    }
}

cout << sum << endl;
return 0;

}