using Mono.Math ;

public class Factorial_BigInteger {
  public static BigInteger iterative ( BigInteger n ) {
    BigInteger total = 1 ;
    if ( n <= 1 ) { return total ; }
    for ( BigInteger i = 2 ; i <= n ; i = BigInteger.Add ( i , 1 ) ) { total = BigInteger.Multiply ( total , i ) ; }
    return total ;
  }
  public static BigInteger iterative ( ulong n ) { return iterative ( new BigInteger ( n ) ) ; }
  public static BigInteger recursive ( BigInteger n ) {
    return n <= 1 ? 1 : n * recursive ( BigInteger.Subtract ( n , 1 ) ) ;
  }
  public static BigInteger recursive ( ulong n ) { return recursive ( new BigInteger ( n ) ) ; }
}