#include <cstdio>
#include <cstring>

int A[30];
char S[1000002];

void solve()
{
    int N = strlen(S),i,last = 45;
    for(i=0; i<N; i++)
        A[S[i] - 'a']++;
    for(i=0; i<N; i++)
        for(int i=0; i<='z'-'a'; i++)
            if(A[i] && i!=last)
            {
                A[last = i]--;
                printf("%c",i+'a');
                break;
            }
}

int main()
{
    freopen("ordine.in","r",stdin);
    freopen("ordine.out","w",stdout);
    gets(S);
    solve();
}