#include <iostream>
#include <cstdlib>
using namespace std;

int main()
{
	int numBooks = 0;               // # of books for this order
	int totalNumBooks = 0;          // total # of books ordered by customer
	bool userWantsToExit = false;   // flag variable to exit while loop

	while (userWantsToExit == false)    // or while (!userWantsToExit)
	{	
		cout << "Enter number of books (-1 to quit): ";
		cin >> numBooks;

		if (numBooks == -1)             // -1 is sentinel value to exit loop
		{
			userWantsToExit = true;
		}
		else if (numBooks >= 0 && numBooks <= 10)
		{
			totalNumBooks += numBooks;
		}
		else
		{
			cout << "Invalid input." << endl;
		}

	}

	cout << "The total number of books ordered is " << totalNumBooks << endl;
		
	system("pause");
	return 0;
}// end of main