#include <iostream>
using namespace std;

int input;

unsigned long long int factorial(int n);

int main()
{
	do
	{
		cout << "Enter a number: ";
		cin >> input;
		if(input < 0)
			cout << "Error - Enter a positive number." << endl;
	} while(input < 0);

	cout << input << "! = " << factorial(input) << endl;

	return 0;
}

unsigned long long int factorial(int n)
{
	if(n <= 1)
		return 1;
	else
		return n*factorial(n-1);
}