#include<stdio.h>  
	 #include<fstream.h>
	#include<string.h>
     
   int v[255],i,j,k,l,m,n;  
   char c,ant,ok;  
     
   int main(){  
     
     
   ifstream f("ordine.in");  
   freopen("ordine.out","w",stdout);  
     
     
	while(f>>c){

		 v[int(c)]++;

			  n++;}

	
  for(i=1;i<=n;i++){
   ok=0;  
       for(j=(int('a'));j<=(int('z'));j++)  
           if(v[j]==((n-i+1)/2+1)){ok=char(j);break;}  
       if(ok){printf("%c",ok);v[int(ok)]--;ant=ok;}  
       else  
           for(j=int('a');j<=int('z');j++)  
               if(v[j]&&j!=ant){printf("%c",char(j));v[j]--;ant=char(j);break;}  
             
         
   }  
   return 0;}