import java.io.*;

public class FactorialMain
{
	public static void main (String[ ] args)
	{
		final int MAX = 20; // because 21! > Long.MAX_VALUE

		final int SENTINEL = -1;

		final String PROMPT = "\nPlease enter a non-negative integer (at most " +
					    MAX + "), or " + SENTINEL + " to quit: ";

		final String ERROR = "Error: the input must be a non-negative integer or " + SENTINEL;

		final String MESSAGE = " factorial is ";

		BufferedReader reader = new BufferedReader (new InputStreamReader (System.in));

		int n;

		while (true)
		{
			try
			{
				System.out.print (PROMPT);
				n = Integer.parseInt (reader.readLine());
				if (n == SENTINEL)
					break;
				if (n > MAX)
					throw new IllegalArgumentException();				
				System.out.println (n + MESSAGE + factorial (n));
			} // try
			catch (Exception e)
			{
				System.out.println (e);
			} // catch
		} // while		
	} // method main

	/**
	 * Calculates the factorial of a non-negative integer, that is, the product of all
	 *  integers between 1 and the given integer, inclusive.  The worstTime(n) is O(n), 
	 *  where n is the given integer.	 
	 *
	 *  @param n the non-negative integer whose factorial is calculated.
	 *
	 *  @return the factorial of n
	 *
	 *  @throws IllegalArgumentException if n is less than 0.
	 *
	 */
	public static long factorial (int n) 
	{
	       	if (n < 0)
        	      	throw new IllegalArgumentException( ); 
       		if (n <= 1)
              		return 1; 
       		return n * factorial (n - 1);
	} // method factorial

} // class FactorialMain