#include <iostream.h>
#include <iomanip.h>


double cost_of_shirts (int shirts) {

  // each pair of shrts bought costs $18, each extra shirt costs $10
  return ((shirts / 2) * 18) + ((shirts % 2) * 10);
}


int main () {

  double cost, cost_of_all_orders = 0;
  int shirts, total_shirts_sold = 0;

  // get this out of the way (all output requires the same options)
  cout << setiosflags (ios::fixed | ios::showpoint) << setprecision(2);

  for (;;) {

    cout << "Enter number of shirts (0 or negative to terminate): ";
    cin >> shirts;

    if (shirts <= 0) {
      break;
    }

    cost = cost_of_shirts (shirts);
    cout << "The cost is $" << cost << endl;

    cost_of_all_orders += cost;
    total_shirts_sold += shirts;

  }

  if (total_shirts_sold != 0) { // play it safe
    cout << "Shirts sold at an average price of $"
         << cost_of_all_orders / total_shirts_sold << endl;
  } // else not doing anything is a reasonable action

}