#include <fstream>
#include <cstring>

using namespace std;

char a[1000005], c[1000005];
int b[30];

int main()
{
    int n, i, j, ok;
    char ch;
    ifstream fin("ordine.in");
    fin >> a;
    fin.close();
    n = strlen(a);
    for(i = 0; i < n; i++)
        b[a[i] - 'a']++;
    for(i = 0; i <= n; i++)
    {
        ok = 1;
        for(j = 0; j <= 27; j++)
        {
            if(b[j] == (n - i) / 2 + 1)
            {
                c[i] = 'a' + j;
                b[j]--;
                ok = 0;
            }
        }
        for(j = 0; j <= 27 && ok; j++)
        {
            if(b[j])
            {
                ch = 'a' + j;
                if(ch != c[i - 1])
                {
                    c[i] = ch;
                    b[j]--;
                    ok = 0;
                }
            }
        }
    }
    ofstream fout("ordine.out");
    for(i = 0; i < n; i++)
        fout << c[i];
    fout.close();
    return 0;
}