#include <iostream>
#include <cstdlib>	//for exit, EXIT_SUCCESS, EXIT_FAILURE
using namespace std;

void factorial(int n);

int main()
{
	factorial(4);
	int i = 5;
	factorial(i);	//Create a copy of i and pass it to the function.
	return EXIT_SUCCESS;
}

void factorial(int n)
{
	if (n < 0) {
		cerr << "argument " << n << " can't be negative.\n";
		exit(EXIT_FAILURE);   //outside of main, exit instead of return
	}

	int f = 1;

	for (int i = 1; i <= n; ++i) {
		f *= i;
	}

	cout << "The factorial of " << n << " is " << f << ".\n";
}