import java.util.Scanner;

public class LongestBlockInString {
    public static void main(String[] args){
        Scanner scanner = new Scanner(System.in);

        String input = scanner.nextLine();
  //      System.out.print(input.length());

        int maxCount = 0;
        char maxSequentChar = ' ';
        int count = 1;

        if(input.length() <= 1){
            System.out.println(input);
        }

        for (int i = 0; i < input.length() - 1; i++){
            if(input.charAt(i) == input.charAt(i + 1)){
                count++;
            }else {
                count = 1;
            }
            if(count > maxCount){
                maxCount = count;
                maxSequentChar = input.charAt(i);
            }
        }
        for(int i = 0; i < maxCount; i++){
            System.out.print(maxSequentChar);
        }

    }
}