Problem:
Score : 300 points

Problem Statement

1000000000000001 dogs suddenly appeared under the roof of Roger's house, all of which he decided to keep. The dogs had been numbered 1 through 1000000000000001, but he gave them new names, as follows:

the dogs numbered 1,2,\cdots,26 were respectively given the names a, b, ..., z;

the dogs numbered 27,28,29,\cdots,701,702 were respectively given the names aa, ab, ac, ..., zy, zz;

the dogs numbered 703,704,705,\cdots,18277,18278 were respectively given the names aaa, aab, aac, ..., zzy, zzz;

the dogs numbered 18279,18280,18281,\cdots,475253,475254 were respectively given the names aaaa, aaab, aaac, ..., zzzy, zzzz;

the dogs numbered 475255,475256,\cdots were respectively given the names aaaaa, aaaab, ...;

and so on.

To sum it up, the dogs numbered 1, 2, \cdots were respectively given the following names:

a, b, ..., z, aa, ab, ..., az, ba, bb, ..., bz, ..., za, zb, ..., zz, aaa, aab, ..., aaz, aba, abb, ..., abz, ..., zzz, aaaa, ...

Now, Roger asks you:

"What is the name for the dog numbered N?"

Constraints

N is an integer.

1 \leq N \leq 1000000000000001

Input

Input is given from Standard Input in the following format:

N

Output

Print the answer to Roger's question as a string consisting of lowercase English letters.

Sample Input 1

2

Sample Output 1

b

Sample Input 2

27

Sample Output 2

aa

Sample Input 3

123456789

Sample Output 3

jjddja

Solution in C# (Mono-mcs 6.8.0.105) (judge verdict: Accepted)

using System;
using System.Collections;
using System.IO;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Collections.Generic;
using System.Threading.Tasks;

class Program
{
	//英小文字の配列
	static readonly char[] alphabet = new char[26]{'a','b','c','d','e','f','g',
																		'h','i','j','k','l','m','n','o','p','q',
																		'r','s','t','u','v','w','x','y','z'};

	//文字数の転換点の配列
	static long[] multiArr;
	//26^nを保存
	static long[] powerArr;
	//各桁を格納する配列
	static int[] alphaIndex;

	//Main
	static void Main(string[] args)
	{
		//初期化
		multiArr = new long[11];
		powerArr = new long[11];
		long sum = 1;
		long inc = 1;
		multiArr[0] = 1;
		powerArr[0] = 1;
		for(int i=1; i<11; i++){
			inc *= 26;
			sum += inc;
			multiArr[i] = sum;
			powerArr[i] = inc;
		}

		//読み込む
		long N = long.Parse(Console.ReadLine());

		//まず、全体の文字数を取得
		int len = 0;
		for(int i=0; i<10; i++){
			if(multiArr[i] <= N && N < multiArr[i+1]){
				len = i+1;
				break;
			}
		}
		if(len == 0){
			len = 11;
		}

		alphaIndex = new int[len];
		//順番にインデックスを埋める
		long tempo = N - multiArr[len-1];
		long rem = 0;
		long div = 0;
		long pow = len-1;
		for(int i=0; i<len; i++){
			div = tempo / powerArr[pow];
			rem = tempo % powerArr[pow];
			alphaIndex[i] = (int)div;
			tempo = rem;
			pow --;
		}

		//答えを作成
		string ans = "";
		StringBuilder sb = new StringBuilder("");
		foreach(int idx in alphaIndex){
			sb.Append(alphabet[idx]);
		}
		ans = sb.ToString();

		/*debug
		foreach(long num in multiArr){
			Console.WriteLine(num);
		}
		foreach(long num in powerArr){
			Console.WriteLine(num);
		}
		*/

		Console.WriteLine(ans);
	}
}