import java.util.Scanner;

public class Task1297 {
    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);

        String input = scanner.nextLine();
        System.out.println(getLongestPalindrome(input));
    }

    public static String getLongestPalindrome(String input) {

        int maxLength = 0;
        String output = "";

        for (int i = 0; i < input.length(); i++) {
            for (int j = i; j < input.length(); j++) {
                String current = input.substring(i, j + 1);
                if (maxLength < current.length() && isPalindrome(current)) {
                    output = current;
                    maxLength = current.length();
                }
            }
        }
        return output;
    }

    public static boolean isPalindrome(String test) {

        char[] array = test.toCharArray();
        if (array.length == 1) return true;
        boolean result = true;

        for (int i = 0; i <= array.length / 2; i++) {
            if (array[i] != array[array.length - 1 - i]) {
                return false;
            }
        }
        return true;
    }
}