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

public class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
		Scanner in = new Scanner(System.in);
		int n = in.nextInt();
		String s = in.next();
		char res = 0;
		for(int i = 0; i < n; i++)
			res ^= s.charAt(i);
		if (res == 0)
			System.out.println("Ok");
		else 
			System.out.printf("%c\n",res);
	}
}