botanics

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Numerics;

namespace Q10212
{
    class Program
    {
        static private int[] iarr; static private char[] par = { ' ' }; static private BigInteger sum;
        static void Main(string[] args)
        {
            for (; ; )
            {
            labelnm: Console.Write("n m>>>");
                try
                {
                    iarr = Array.ConvertAll<string, int>(Console.ReadLine().Split(par, StringSplitOptions.RemoveEmptyEntries), int.Parse);
                    if (iarr.Length != 2 || iarr[0] < iarr[1]) throw new Exception();
                }
                catch { goto labelnm; }
                sum = 1;
                for (int i = 0; i < iarr[1]; i++)
                    sum *= (iarr[0] - i);
                for (int i = 0; i < sum.ToString().Length; i++)
                {
                    if (sum.ToString().Substring(sum.ToString().Length - i - 1, 1) != "0")
                    {
                        Console.WriteLine("Ans={0}", sum.ToString().Substring(sum.ToString().Length - i - 1, 1));
                        break;
                    }
                }
            }
        }
    }
}