import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String str = scanner.nextLine(); // 从用户输入读取字符串
        System.out.println(find(str));
    }

    public static String find(String str) {
        int[] frequency = new int[26]; // 用于记录每个字母的出现次数，假设只考虑大写字母

        // 统计每个字母的出现次数
        for (int i = 0; i < str.length(); i++) {
            char ch = str.charAt(i);
            if (Character.isUpperCase(ch)) { // 只考虑大写字母
                frequency[ch - 'A']++;
            }
        }

        // 找到出现次数最多的字母
        int maxFrequency = 0;
        for (int i = 0; i < frequency.length; i++) {
            maxFrequency = Math.max(maxFrequency, frequency[i]);
        }

        // 构建结果字符串
        StringBuilder result = new StringBuilder();
        for (char ch = 'A'; ch <= 'Z'; ch++) {
            if (frequency[ch - 'A'] == maxFrequency) {
                result.append(ch);
            }
        }

        return result.toString();
    }
}

`