Valid Usernames

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

namespace ValidUsernames
{
    class ValidUsernames // Alexander Anguelov
    {
        static void Main(string[] args)
        {
            string input = Console.ReadLine();
            string userName = @"\b\w{3,25}\b";
            MatchCollection matches = Regex.Matches(input, userName);
            List<string> valid = new List<string>();
            foreach (var match in matches)
            {
                if (char.IsLetter(match.ToString().First()))
                {
                    valid.Add(match.ToString());
                }
            }
            int biggestSum = 0;
            int pos = 0;
            for (int i = 0; i < valid.Count - 1; i++)
            {
                int currSum = valid[i].Length + valid[i + 1].Length;
                if (currSum > biggestSum)
                {
                    biggestSum = currSum;
                    pos = i;
                }
            }
            Console.WriteLine(valid[pos]);
            Console.WriteLine(valid[pos + 1]);
        }
    }
}