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

int factorial(int n);

int main()
{
	cout << "The factorial of 4 is " << factorial(4) << ".\n";
	int i {factorial(5)};
	cout << "The factorial of 5 is " << i << ".\n";
	return EXIT_SUCCESS;
}

int factorial(int n)
{
	if (n < 0) {
		cerr << "argument " << n << " can't be negative.\n";
		exit(EXIT_FAILURE);
	}

	int f {1};

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

	return f; //Create a copy of the value of f and return it to the caller.
}