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

#define MAX_N 1000005
#define FIN "ordine.in"
#define FOUT "ordine.out"

int N, cnt[26], ns;
char S[MAX_N], sol[MAX_N], pos[MAX_N];

int main(void)
{
    int i, prev;

    freopen(FIN, "r", stdin);
    freopen(FOUT, "w", stdout);

    fgets(S, sizeof(S), stdin);
   
    N = strlen(S)-1;
    for (i = 0; i < N; ++i)
        ++cnt[S[i]-'a'];
    for (prev = -1; N; --N)
    {
        for (i = 0; i < 26; ++i)
            if (cnt[i] && i != prev) break;
        if (i == 26) break;
        sol[ns++] = i+'a';
        --cnt[prev = i];
    }
    for (i = ns-2; N && i >= 0; --i)
        if (sol[i] != prev+'a' && sol[i+1] != prev+'a')
            pos[i] = 1, --N;
    for (i = 0; i < ns; ++i)
    {
        printf("%c", sol[i]);
        if (pos[i]) printf("%c", prev+'a');
    }
    printf("\n");

    return 0;
    
}