#include <cstdio>
#include <cstring>

#define Lmax 1000001
#define alfa 26

char S[Lmax], f[alfa];

int main() {
    freopen("ordine.in", "r", stdin);
    freopen("ordine.out","w",stdout);
    
    scanf("%s", S);
    int L = strlen(S);
    for (int i=0; i<L; ++i)
        ++f[S[i]-'a'];
    
    int last = -1;
    for (int i=0; i<L; ++i) {
        int j;
        for (j=0; j<26; ++j)
            if (f[j] == (L-i)/2+1) {
                printf("%c", j+'a');
                last = j;
                --f[j];
                break;
            }
        if (j<26) continue;
        for (j=0; j<26; ++j)
            if (f[j] && j!=last) {
                last = j;
                printf("%c", j+'a');
                --f[j];
                break;
            }
    }
    
    return 0;
}