import java.util.Scanner;

 //输入字符串，去掉重复的字符，并按ASSIC码值排序

 public class quChong {

     public static void main(String arg[]){

         Scanner cin = new Scanner(System.in);

         String str = cin.nextLine();

         char c[] = str.toCharArray();

         int count=0;

         int flag=0;

         for(int i=0;i<c.length-count;i++){

             char temp;

             if(count>i&&flag==1){

                 i=0;

             }else if(count<=i&&flag==1){

                 i=i-count;

             }

             temp = c[i];

             flag = 0;

             for(int j =i+1;j<c.length-count;j++){

                 if(temp==c[j]){

                     for(int z =i;z<c.length-count-1;z++){

                         c[z]=c[z+1];

                     }

                     count++;

                     flag=1;

                     break;

                 }

             }

         }

         for(int i=0;i<c.length-count;i++){

             for(int j=0;j<c.length-count-1;j++){

                 char temp;

                 if(c[j]>c[j+1]){

                     temp=c[j];

                     c[j]=c[j+1];

                     c[j+1]=temp;

                 }

             }

         }

         for(int i=0;i<c.length-count;i++){

             System.out.print(c[i]);

         }

     }

 }