#include <cstdio>
using namespace std;

const int MAXSIR = 1000010;
const int MAXALFABET = 'z' - 'a' + 10;

int vf[MAXALFABET];
int max = -1,min = MAXALFABET;
int l_sir = 0;

void citire()
{
    char sir[MAXSIR] = {0};

    freopen("ordine.in","r",stdin);
    freopen("ordine.out","w",stdout);
    gets(sir + 1);
    for (int i = 1;sir[i] != 0;++i)
    {
        ++l_sir;
        ++vf[sir[i] - 'a' + 1];
        if (sir[i] - 'a' + 1 > max)
            max = sir[i] - 'a' + 1;
        if (sir[i] - 'a' + 1 < min)
            min = sir[i] - 'a' + 1;
    }
}

void prelucrare()
{
    char sir[MAXSIR] = {0};
    int l_sir_2 = 0;
    int poz_afisat = -1;
    bool gasit;
    for (int i = 1;i <= l_sir;++i)
    {
        gasit = false;
        for (int j = min;j <= max;++j)
            if (vf[j] == (l_sir - i + 1) / 2 + 1 && j != poz_afisat)
            {
                poz_afisat = j;
                sir[++l_sir_2] = 'a' + j - 1;
                --vf[j];
                while (vf[min] == 0)
                    ++min;
                gasit = true;
                break;
            }
        if (!gasit)
        {
            for (int j = min;j <= max;++j)
                if (vf[j] > 0 && j != poz_afisat)
                {
                    poz_afisat = j;
                    sir[++l_sir_2] = 'a' + j - 1;
                    --vf[j];
                    while (vf[min] == 0)
                        ++min;
                    break;
                }
        }
    }
    puts(sir + 1);
}

int main()
{
    citire();
    prelucrare();
    return 0;
}