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: Wrong Answer)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.IO;

public class MainClass
{
    public static void Main(string[] args)
    {
        var sw = new StreamWriter(Console.OpenStandardOutput(), Console.Out.Encoding);
        sw.AutoFlush = false;
        Console.SetOut(sw);

        if (args.Length > 0 && args[0] == "debug")
        {
            var inputs = new[] { @"
2
				", @"
27
                ", @"
123456789
				", @"

				", @"

				" };

            foreach (var str in inputs)
            {
                var text = str.TrimStart();
                if (text.Length == 0) continue;
                var reader = new System.IO.StringReader(text);
                Console.SetIn(reader);
                Main2(args);
                Console.WriteLine("----------------------------------");
            }
        }
        else
            Main2(args);

        Console.Out.Flush();
    }

    private static void Main2(string[] args)
    {
        checked
        {
            //var MAX = 1000000000000001L;
            var N = long.Parse(Console.ReadLine());

            var res = "";
            while (N > 0)
            {
                var s = N % 26;
                res = (char)('a' + s - 1) + res;
                N /= 26;
            }
            Console.WriteLine(res);
        }
    }

    #region "入力"

    private static string NextInput = "";

    private static int Int() => int.Parse(Str());
    private static long Long() => long.Parse(Str());
    private static float Float() => float.Parse(Str());
    private static double Double() => double.Parse(Str());
    private static string Str()
    {
        var s = NextInput;
        if (s == "") s = Console.ReadLine();

        var idx = s.IndexOf(' ');
        if (idx >= 0)
        {
            NextInput = s.Substring(idx + 1);
            return s.Substring(0, idx);
        }
        NextInput = "";
        return s;
    }

    #endregion

}

public static class EnumerablePlus
{
    ///<summary>出力文字列に変換。</summary>
    public static string MkString<T>(this IEnumerable<T> list)
    {
        return list.Aggregate(
            new System.Text.StringBuilder(),
            (a, b) => ((a.Length > 0) ? a.Append(' ') : a).Append(b)
            ).ToString();
    }

    ///<summary>添字付きfor。</summary>
    public static void ForEachWithIdx<T>(this IEnumerable<T> list, Action<T, long> a)
    {
        long i = 0;
        foreach (var item in list)
        {
            a(item, i++);
        }
    }

    ///<summary>添字なしfor。</summary>
    public static void ForEach<T>(this IEnumerable<T> list, Action<T> a)
    {
        foreach (var item in list)
        {
            a(item);
        }
    }

    ///<summary>指定回数デリゲートを呼び出す。</summary>
    public static IEnumerable<T> Repeat<T>(long n, Func<T> f)
    {
        for (long i = 0; i < n; i++)
        {
            yield return f();
        }
    }

    ///<summary>条件付きMin。</summary>
    public static T MinBy<T, U>(this IEnumerable<T> xs, Func<T, U> key) where U : IComparable<U>
    {
        return xs.Aggregate((a, b) => key(a).CompareTo(key(b)) <= 0 ? a : b);
    }

    ///<summary>条件付きMax。</summary>
    public static T MaxBy<T, U>(this IEnumerable<T> xs, Func<T, U> key) where U : IComparable<U>
    {
        return xs.Aggregate((a, b) => key(a).CompareTo(key(b)) >= 0 ? a : b);
    }

    ///<summary>条件付きMax。複数の値を返す。</summary>
    public static List<T> MaxByList<T, U>(this IEnumerable<T> xs, Func<T, U> key) where U : IComparable<U>
    {
        return xs.Aggregate(
            new List<T>(),
            (list, b) =>
            {
                if (list.Count == 0) { list.Add(b); return list; }

                var aa = key(list[0]);
                var bb = key(b);

                if (aa.CompareTo(bb) < 0) list.Clear();

                if (aa.CompareTo(bb) <= 0) { list.Add(b); return list; }

                return list;
            });
    }

    ///<summary>複数の値を返す。</summary>
    public static List<T> MaxList<T>(this IEnumerable<T> xs) where T : IComparable<T>
    {
        return xs.MaxByList(o => o);
    }
}

public static class MathPlus
{
    public static long Power(long x, long n)
    {
        if (n == 0) return 1;
        if (n % 2 == 0) return Power(x * x, n / 2);
        return x * Power(x, n - 1);
    }

    ///<summary>約数。順番は不定。</summary>
    public static List<long> Divisor(long n)
    {
        var list = Prime(n).
            GroupBy(id => id).
            Select(pair => new { prime = pair.Key, power = pair.Count() }).
            ToList();

        var result = list.Aggregate(new List<long>(), (left, pair) =>
        {
            var list2 = Enumerable.
                Range(0, pair.power + 1).
                Aggregate(new List<long>(), (res, l) =>
                {
                    var next = res.Count > 0 ? res.Last() * pair.prime : 1L;
                    res.Add(next);
                    return res;
                });

            if (left.Count == 0) return list2.ToList();

            return list2.
                SelectMany(l => left.Select(m => l * m)).
                ToList();
        });

        return result;
    }

    ///<summary>順列の数。</summary>
    public static long Permutation(long n, long r)
        => Factorial(n, r);

    ///<summary>組合せの数。</summary>
    public static long Combination(long n, long r)
    {
        if (r == 0) return 1;

        return Factorial(n, r) / Factorial(r);
    }

    ///<summary>階乗。カウントも指定可能。</summary>
    public static long Factorial(long a, long count = long.MaxValue)
    {
        long res = 1;

        for (long i = 0; a > 0 && i < count; i++)
        {
            checked { res *= a; }
            --a;
        }

        return res;
    }

    ///<summary>素因数分解。昇順。</summary>
    public static List<long> Prime(long a)
    {
        var res = new List<long>();

        var a2 = Math.Ceiling(Math.Sqrt(a));
        for (long b = 2; b <= a2; ++b)
        {
            while (a % b == 0)
            {
                res.Add(b);
                a = a / b;
                a2 = Math.Ceiling(Math.Sqrt(a));
            }
        }

        if (a > 1) res.Add(a);
        return res;
    }

    ///<summary>最小公倍数。</summary>
    public static long Lcm(IEnumerable<long> list)
    {
        return list.Aggregate((a, b) => Lcm(a, b));
    }

    ///<summary>最小公倍数。</summary>
    public static long Lcm(long a, long b)
    {
        return checked(a * b) / Gcd(a, b);
    }

    ///<summary>最大公約数。</summary>
    public static long Gcd(IEnumerable<long> list)
    {
        return list.Aggregate((a, b) => Gcd(a, b));
    }

    ///<summary>最大公約数。</summary>
    public static long Gcd(long a, long b)
    {
        if (a < b)
            Gcd(b, a);

        while (b != 0)
        {
            var rem = a % b;
            a = b;
            b = rem;
        }

        return a;
    }

    ///<summary>整数の切り上げ。</summary>
    public static long Ceiling(long a, long b) => (a + (b - 1)) / b;

    ///<summary>区間内の合計。beginも１から？end含む。</summary>
    public static ulong Sum(ulong begin, ulong end)
    {
        var up = Sum(end);
        var low = Sum(begin - 1);
        return up - low;
    }

    ///<summary>1からの合計。</summary>
    public static ulong Sum(ulong end) => checked((end + 1) * end / 2);
}

public static class Util
{
    ///<summary>リスト内の値を移動する。</summary>
    public static void Move<T>(this IList<T> list, int from, int to)
    {
        var item = list[from];

        for (var idx = from; idx > to; --idx)
        {
            list[idx] = list[idx - 1];
        }

        list[to] = item;
    }

    ///<summary>最後の値か、デフォルト値を取得。</summary>
    public static T LastOrValue<T>(this IList<T> list, T value)
    {
        return list.Count > 0 ? list.Last() : value;
    }

    ///<summary>辞書をMultiSetとして使うときのAdd。</summary>
    public static void AddMulti<T>(this IDictionary<T, int> xs, T key)
    {
        if (xs.ContainsKey(key))
            xs[key]++;
        else
            xs.Add(key, 1);
    }

    ///<summary>値を交換する。</summary>
    public static void Swap(ref int x, ref int y)
    {
        int temp = x;
        x = y;
        y = temp;
    }

    public static void Assert(bool cond)
    {
        if (!cond) throw new Exception();
    }
}