calculate the sum of diagonals in a matrix

Question

I need to calculate the sum of two diagonals in a matrix in C++, I already have a solution for that but I must be dumb because I cant understand what it is doing, so I would

Answer 1

int num[5][5]={0}; //decleration
int i=0,j=0,sum=0; 
for (int i=0;i<5;i++)
{
    for (int j=0;j<5;j++)
    {
        cin>>num[i][j];
    }                          //Taking Matrix input
}
        cout<