#include <iostream> 
#include <string> 
using namespace std; 
 
void sort(string s) 
{ 
  char tmp[100]; 
  int len=s.size(); 
  int count=0,i,j; 
  for (i=0;i<len;i++) 
  { 
   for (j=i+1;j<len;j++) 
   { 
    if (s[i]==s[j]) 
    { 
     s[j]='0'; 
    } 
   } 
 
  } 
 
  for (i=0;i<len;i++) 
  { 
   if (s[i]>='a' && s[i]<='z') 
   { 
    tmp[count++]=s[i]; 
   } 
  } 
 
  //冒泡排序 
  for (i=0;i<count;i++) 
  { 
   for (j=0;j<i;j++) 
   { 
    char temp; 
    if (strcmp(&tmp[j],&tmp[i])>0) 
    { 
     temp=tmp[j]; 
     tmp[j]=tmp[i]; 
     tmp[i]=temp; 
    } 
   } 
  } 
 
 
  for (i=0;i<count;i++) 
  cout<<tmp[i]; 
 
  cout<<endl; 
} 
 
void main() 
{ 
 string s; 
 cin>>s; 
 sort(s); 
}