MoreRegex/5.OnlyLetters

using System;
using System.Text.RegularExpressions;
using System.Linq;
using System.Globalization;
using System.Collections.Generic;
using System.Text;

public class Example

{
    public static void Main(string[] args)

    {
      string input = Console.ReadLine();

   string pattern = @"(\d+)([A-Za-z])";

    MatchCollection m = Regex.Matches(input,pattern);
    StringBuilder result = new StringBuilder(input);

    foreach (Match currM in m)
    {
      result.Replace(currM.Groups[1].Value,currM.Groups[2].Value);
    }

      Console.WriteLine(result);
    }
}