StringStuff

        static void Main(string[] args)
        {
            char character = 'a';
            int nextIndex = 0, wordsCount, characterCount = 0;
            string lastLetters = "", sentence, lastLettersCopy;
            sentence = Console.ReadLine();
            while (nextIndex != -1)
            {
                nextIndex = sentence.IndexOf(" ", nextIndex) + 1;
                if (nextIndex == 0)
                    nextIndex--;
                else
                    lastLetters += sentence[nextIndex - 2];
            }
            lastLetters += sentence[sentence.Length - 2];
            for (int i = 0; i < 26; i++)
            {
                lastLettersCopy = lastLetters;
                lastLettersCopy = lastLettersCopy.Replace(((char)('a' + i)).ToString(), "");
                if (lastLetters.Length - lastLettersCopy.Length > characterCount)
                {
                    characterCount = lastLetters.Length - lastLettersCopy.Length;
                    character = (char)('a' + i);
                }
            }
            Console.WriteLine(character);
        }