import java.util.*;
import java.lang.*;

class Main
{
    static final String in = "aabbccddeeffgghhiijjkaaaabbbbccccddddeeeeffffggggh";

    public static void main (String[] args) throws java.lang.Exception
    {
        System.out.print(firstLonelyChar(in));
	}

    static public String firstLonelyChar(String input)
    {
        while(input.length() > 0)
        {
            int curLength = input.length();
            String first = String.valueOf(input.charAt(0));
            input = input.replaceAll(first, "");
            if(input.length() == curLength - 1)
                return first;
        }
        return null;
    }	
}