How to add the diagonal elements in an array?

For k = 4 I need to add the highlighted elements.If user enters k = 4 then the first element will be in the fourth row , second element in third row and so on.
k=0| 1
k=1| 1 1 -----> fourth from here
k=2| 1 2 1 -----> third from here
k=3| 1 3 3 1 -----> second element from here
k=4| 1 4 6 4 1 -----> first element from this row ,how do I add all these elements The code for Pascal's triangle is

#include<iostream>
using namespace std;

int main()
{
   int pascal = 1;
   int rows, columns;
   cout << "Enter the number of rows : ";
   cin >> rows;
   cout << endl;

   int **array = new int *[rows];     //generating array
   for (int i = 0; i <= rows; i++)
       array[i] = new int[columns];

   for (int i = 0; i < rows; i++)    // cycle to show pascal's triangle
   {
        int val = 1;
        for (int j = 1; j < (rows - i); j++)
        {
            cout << "  ";
        }
        for (int k = 0; k <= i; k++)
        {
            cout << "   " << val;
            val = val * (i - k) / (k + 1);
        }
        cout << endl;
    }
    cout << endl;

    int k, sum;
    cout << "Enter the value k: ";
    cin >> k;
    for (int i = 0; i <= rows; i++) {  //cycle to calculate `kth` `fibonacci number`
        for (int j = 0; j <= i; j++) {
            if (j == 0 || i == 0) {
                pascal = 1;  
            }
            else {
                pascal = pascal *(i - j + 1) / j; 
            }
            array[i][j] = pascal; 
            sum = sum + array[k - i][i]; //trying to sum the diagonal elements
        }
        cout << "\n";
     }
     cout << sum;
     return 0;
 }