#include<stdio.h>
char c[1000001];
int i,j,a[123],aux,nr;
int main()
{  freopen("ordine.in","r",stdin);
   freopen("ordine.out","w",stdout);
   scanf("%s",c);
   for(i=0;c[i];i++)  ++a[c[i]];
   for(i=95;i<=122;i++)   while(a[i]) { if(i!=aux) { printf("%c",i);
                                                     aux=i;
                                                     --a[i];
                                                   }
                                       else { j=i+1;
                                              while(!a[j]&&j<122) j++;
                                              printf("%c",j);
                                              aux=j;
                                              --a[j];
                                            }
                                    }
   printf("\n");                                 
   fclose(stdin);
   fclose(stdout);
   return 0;
}