aaaicu

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int [] abc = new int [26];
		int max = 0 ;
		String result = "";
		String input = scan.nextLine();
		input = input.toUpperCase();

		for(int i = 0 ; i < input.length(); i ++) {
			abc[input.charAt(i)-'A']++;
		}

		for(int i =0 ; i < abc.length ; i++) {
			if(max == abc[i]) {
				result = "?";
			}else if (max < abc[i]) {
				max = abc[i];
				result = Character.toString((char) ('A'+i));
			}
		}		

		System.out.println(result);
		scan.close();		
	}
}