using System;

using System.Text;

public class Program

{

    public static void buildLowestNumber(string str, int n, StringBuilder res) 

    { 

        // If there are 0 characters to remove from str, 

        // append everything to result 

        if (n == 0) 

        { 

            res.Append(str); 

            return; 

        }  

        int len = str.Length - 1; 

​

        // If there are more characters to remove than string 

        // length, then append nothing to result 

        if (len <= n) 

            return; 

​

        // Find the smallest character among first (n+1) characters 

        // of str. 

        int minIndex = 0; 

        for (int i = 1; i<=n ; i++) 

            if (str[i] < str[minIndex]) 

                minIndex = i;  

        // Append the smallest character to result 

        res.Append(str[minIndex]);  

​

        // substring starting from minIndex+1 to str.length() - 1. 

        string new_str = str.Substring(minIndex+1, len-minIndex); 

​

        // Recur for the above substring and n equals to n-minIndex 

        buildLowestNumber(new_str, n-minIndex, res); 

    } 

    // A wrapper over buildLowestNumberRec() 

    public static string  GenerateLowestNumber(string number, int n) 

    { 

        StringBuilder res = new StringBuilder(); 

​

        // Note that result is passed by reference 

        buildLowestNumber(number, n, res); 

​

        return res.ToString(); 

    }           

    public static void Main()

    {

        string str = "216504"; 

        int n = 3; 

        Console.WriteLine(GenerateLowestNumber(str, n));

    }

}