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# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Linq;

namespace AtCoder
{
    static class Program_D
    {
        private class Solver
        {

            private long baseNum = 26;
            // 149-C - One Quadrillion and One Dalmatians
            public void Solve()
            {
                var n = long.Parse(Console.ReadLine())-1;
                var keta = CalcKeta(n);

                var a = CreateArray(keta.Item1, keta.Item2);
                var str = new string(a.Select(tmp => (char)('a' + tmp)).ToArray());

                Console.WriteLine(str);

                //TestKeta();
                //TestBaseArray();
            }

            private void TestBaseArray()
            {
                var vals = new int[] {0, 26 - 1, 26, 27, 28, 26*26 - 1, 26*26, 26*26 + 1, 26*26*26 - 1, 26*26*26};
                foreach (var val in vals)
                {
                    var a = CreateArray(4, val);
                    var str = new string(a.Select(tmp => (char) ('a' + tmp)).ToArray());
                    Console.WriteLine($"{val} : {str}");

                }
            }

            private void TestKeta()
            {
                var vals = new int[] {0, 25, 26, 702-1, 703-1, 18278-1, 18279-1};
                foreach (var val in vals)
                {
                    var keta = CalcKeta(val);
                    Console.WriteLine($"{val} => {keta.Item1}, {keta.Item2}");
                }
            }

            private Tuple<long, long> CalcKeta(long n)
            {
                long test = 0;
                for (long i = 1; i < 100; ++i)
                {
                    long nextTest = test + Pow(baseNum, i);
                    if (n < nextTest)
                    {
                        return Tuple.Create(i, n - test);
                    }
                    test = nextTest;
                }
                return Tuple.Create(-1L, -1L);
            }

            // nthは0オフセット.
            private int[] CreateArray(long keta, long val)
            {
                var result = new int[keta];
                for (int i = 0; i < keta; ++i)
                {
                    result[i] = (int) (val%baseNum);
                    val /= baseNum;
                }
                Array.Reverse(result);
                return result;
            }
            
            private static long Pow(long a, long b)
            {
                long result = 1;
                for (int i = 0; i < b; ++i)
                {
                    result *= a;
                }
                return result;
            }

            private static void Main(string[] args)
            {
                new Solver().Solve();
            }

        }
    }

}