#include <stdio.h>
#include <string.h>

char v[1000001];
int l[27],i,j,a,b,n;

int main()
{
    freopen("ordine.in","r",stdin);
    freopen("ordine.out","w",stdout);
    v[0]=' ';
    fgets(v+1,1000000,stdin);
    n=strlen(v)-2;
    for (i=1;i<=n;++i) ++l[v[i]-(int)'a'+1];
    for (i=1;i<=n;++i)
    {
        j=1;
        while (!l[j]||(j==a)) ++j;
        b=a;
        a=j;
        for (j=a+1;j<27;++j)
            if ((l[j]==(n-i+1)/2+1)&&(j!=b))
                a=j;
        --l[a];
        printf("%c",(char)(a+(int)'a')-1);
    }
    return 0;
}