import java.math.BigInteger;

public class Factorial {
    
    /* returns the n factorial, or -1 if it does not exist; works up to 20 */
    public static long factorial (long n) {
        if (n > 1) return n * factorial(n - 1);
        else if (n >= 0) return 1;
        else return -1;
    }
    
    /* returns the n factorial, or -1 if it does not exist (in the form of BigInteger) */
    public static BigInteger bigFactorial (int n) {
        if (n > 1) return new BigInteger(String.valueOf(new BigInteger(String.valueOf(n)).multiply(bigFactorial(n - 1))));
        else if (n >= 0) return new BigInteger("1");
        else return new BigInteger("-1");
    }
}

              Created: February 12, 2014
Completed in full by: Michael Yaworski