#include<cstdio>
#include<cstring>
#define DIM 1000000
int ch[27];
int main()
{
	freopen("ordine.in","r",stdin);freopen("ordine.out","w",stdout);
	int i,aux=0;char s;
	while(scanf("%c",&s)!=EOF)
		ch[s-96]++;
	for(i=1;i<=26;i++)
		if(ch[i])
		{
			printf("%c",i+96);ch[i]--;
			if(aux)
			{
				i=aux-1;aux=0;continue;
			}
			if(ch[i])
				aux=i;
			else
				aux=0;
		}
}