#include<stdio.h>
#include<iostream.h>

/// -97
long v[26],n,i,poz=-1,last=-1; char s;


int cauta()
{int j,var2; long var=(n-i+1)/2+1;

for(j=25;j>=0;j--)
{if(j!=last)
{var2=j;
if(v[j]==var) {v[j]--; last=j; return j;}}} last=var2; v[var2]--; return var2;


}


main()
{freopen("ordine.in","r",stdin);
 freopen("ordine.out","w",stdout);

 while(!feof(stdin)) {s=fgetc(stdin); v[s-97]++; n++;} n--;

 for(i=1;i<=n;i++)
 printf("%c",cauta()+97);    cout<<endl;


}