using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(FindFirstNonRepeatingCharacter("DEFD"));

    }

    public static char? FindFirstNonRepeatingCharacter(string givenString) {

        for (int i = 0; i < givenString.Length; i++) {

            String letter = givenString[i].ToString();

            if (givenString.IndexOf(letter, StringComparison.OrdinalIgnoreCase) == givenString.LastIndexOf(letter, StringComparison.OrdinalIgnoreCase)) {

                return givenString[i];

            }

        }

        return null;

    }

}