#include<cstdio>
#include<algorithm>
#include<cstring>
#define Nmax 1000005
using namespace std;
int n,i,j,w[30],v1,v2,p;
char s[Nmax];
int main()
{
    freopen("ordine.in","r",stdin);
    freopen("ordine.out","w",stdout);
    gets(s+1);
    n=strlen(s+1);
    for (i=1;i<=n;i++)
    {
        ++w[s[i]-'a'];
    }
    p=-1;
    for (i=1;i<=n;i++)
    {
        v2=-1; v1=-1;
        for (j=0;j<26;j++)
        if (j!=p)
        {
            if (w[j]==1+(n-i+1)/2)
            v2=j;
            else if (w[j] && v1==-1) v1=j;
        }
        if (v2!=-1)
        {
            printf("%c",v2+'a');
            --w[v2];
            p=v2;
        }else
        {
            printf("%c",v1+'a');
            --w[v1];
            p=v1;
        }
    }
    return 0;
}