using System;
using System.Numerics;
using System.Collections.Generic;
 
public static class BigIntExtensions
{
    public static BigInteger Factorial(this BigInteger integer)
    {
        if(integer < 1) return new BigInteger(1);
 
        BigInteger result = integer;
        for (BigInteger i = 1; i < integer; i++)
        {
            result = result * i;
        }
 
        return result;
    }
}
 
public class Test
{
    public static BigInteger CalcCount(BigInteger n, BigInteger r)
    {
        BigInteger result = n.Factorial() / (n - r).Factorial();
        return result;
    }
   
    public static int[] nthPerm(BigInteger myIndex, int n, int r, BigInteger total)
    {
        int j = 0, n1 = n;
        BigInteger temp, index1 = myIndex;
       
        temp = total ;
        List<int> indexList = new List<int>();
       
        for (int k = 0; k < n; k++) {
               indexList.Add(k);
        }
       
        int[] res = new int[r];
 
        for (int k = 0; k < r; k++, n1--) {
            temp /= n1;
            j = (int) (index1 / temp);
            res[k] = indexList[j];
            index1 -= (temp *  j);
            indexList.RemoveAt(j);
        }
       
        return res;
    }
   
    public static void Main()
    {
        int[] testSet = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21};
        BigInteger numPerms, n, r;
        n = testSet.Length;
        r = testSet.Length;
        numPerms = CalcCount(n, r);
        Console.WriteLine(numPerms);
       
        BigInteger testIndex = new BigInteger(1234567890987654321);
        int[] myNthIndex = nthPerm(testIndex, (int) n, (int) r, numPerms);
        int[] myNthPerm = new int[(int) r];
 
        for (int i = 0; i < (int) r; i++) {
               myNthPerm[i] = testSet[myNthIndex[i]];
        }
       
        Console.WriteLine(string.Join(",", myNthPerm));
    }
}