#include <iostream>
using namespace std;

int main() 
{
  int num_items;
  cout << "Enter number: ";
  cin >> num_items;
  if ( num_items > 100000 ) {
    cout << "Number is too large: " << num_items << endl;
    return -1;                  // error exit
  }
  int counts[num_items];
  for(int i = 0; i < num_items; i++)
    counts[i] = i;
  int sum = 0;
  for(int i = 0; i < num_items; ++i)
    sum += counts[i];
  cout << "Sum of 0.." << num_items - 1 << ": " << sum << endl;
  return 0;
}