import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String str = scanner.next();
        String subStr = str;
        for (int i = 0; i < str.length(); i++) {
            for (int j = i + 1; j < str.length(); j++) {
                if (str.charAt(i) == str.charAt(j)){
                    if (str.substring(i,j+1).length() < subStr.length()){
                        subStr = str.substring(i,j+1);
                    }
                }
            }
        }
        System.out.println(subStr);
    }
}