#include<stdio.h>
#include<string.h>

char v[1000002];
int f[30];

int main()
{
    freopen("ordine.in","r",stdin);
    freopen("ordine.out","w",stdout);
    int n,i,j,m;
    char last;
    scanf("%s",&v);
    n=strlen(v);
    for(i=0;i<n;i++)
        f[v[i]-'a'+1]++;
    m=n;
    ///////
    while(m)
    {
        m--;
        for(i=1;i<=27;i++)
            if(f[i]&&(i+'a')!=last)
            {
                printf("%c",i+'a'-1);
                f[i]--;
                last=i+'a';
            }
    }
}