#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;
}
int main()
{
string s;
cin >> s;
sort(s);
}