Find missing element in the series.

Here is the code :

#include <iostream>
#include <conio.h>

using namespace std;

void main()
{
  int arr[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 19, 20 };
  int result = 0;
  int numberOfElements = 20;
  int sum = (numberOfElements * (numberOfElements + 1)) / 2;
  int indivisualSum = 0;

  for (int index = 0; index < numberOfElements - 1; ++index)
  {
    indivisualSum += arr[index];
  }
  
  cout << sum - indivisualSum;

   _getch();
}

Thanks