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;//两层循环使得各种字串情况可以详尽for (int i = 0; i < str.length(); i++) {for (int j = i; j < str.length(); j++) {if (check(str.substring(i,j+1),str)){if (str.substring(i,j+1).length() < subStr.length()){subStr = str.substring(i,j+1);}}}}System.out.println(subStr);}//检查str中的每一个字符是不是都可以在newSubStr中找到public  static boolean check(String newSubStr,String str){for (int i = 0; i < str.length(); i++) {if (newSubStr.indexOf(str.charAt(i)) == -1){return false;}}return true;}
}