#include <stdio.h>
#include <string.h>
#define Nmax 1000001

long c[35], n, i, last=100;
char ch;
char jeg[Nmax];

int main()
{
	freopen("ordine.in", "r", stdin);
	scanf("%s", &jeg);
	n=strlen(jeg);
	for (i=0; i<n; i++) c[jeg[i]-'a']++;
	fclose(stdin);
	freopen("ordine.out", "w", stdout);
	while (n)
	{
		for (i=0; i<=25; i++)
		{
			if (c[i] && i!=last) break;
		}
		last=i;
		c[i]--;
		printf("%c", 'a'+i);
		n--;
	}
	fclose(stdout);
	return 0;
}