#include<stdio.h>
int f[30];
char s[1000005];
int main()
{
	freopen("ordine.in","r",stdin);
	freopen("ordine.out","w",stdout);
	int ant,i,j;
	gets(s);
	for(i=0;s[i];i++)
		f[s[i]-'a']++;
	ant=-1;
	for(i=0;s[i];i++)
	{
		for(j=0;j<26;j++)
			if(f[j] && ant!=j)
			{
				ant=j;
				f[j]--;
				break;
			}
		printf("%c",ant+'a');
	}
	printf("\n");
	return 0;
}