using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("aaabccccdd".ParseInput());

        Console.WriteLine("yyyyyzzzzzzzzzz".ParseInput());

        Console.WriteLine("abcd".ParseInput());

        Console.WriteLine("hhhccchhhccc".ParseInput());

    }

}

​

public static class Extensions

{

    public static string ParseInput(this string input)

    {

        return string.Join("", input.GroupWhile().Select(x => $"{x.Key}{(x.Count > 1 ? x.Count.ToString() : null)}"));

    }

​

    public static IEnumerable<GroupedList<T>> GroupWhile<T>(this IEnumerable<T> items) where T : struct

    {

        var res = new List<GroupedList<T>>();

​

        if (items == null)

            goto End;

​

        Nullable<T> prev = null;

        GroupedList<T> innerList = null;

​

        foreach (T i in items)

        {

            if (!i.Equals(prev))

            {

                innerList = new GroupedList<T>(i);

                res.Add(innerList);

            }

​

            innerList.Add(i);

            prev = i;

        }

​

        End:

        return res;

    }

}

​

public class GroupedList<T> : List<T>

{

    public readonly T Key;

​

    public GroupedList(T key) => (Key) = (key);

}