#include<stdio.h>
#define ALPHA 27
#define N 1000004

int n,v[ALPHA];

void cit()
{
	int i;
	char c[N];
	fgets( c,N,stdin );
	for( i=0 ; c[i]!=10 ; ++i )
		++v[c[i]-'a'];
	n=i;
}

void solve()
{
	int ex=-1,i;
	while( n-- )
	{
		for( i=0 ; i<ALPHA ; ++i )
			if( v[i]!=0 && i!=ex )
				break;
		ex=i;
		printf("%c",ex+'a');
		--v[ex];
	}
	printf("\n");
}

int main()
{
	freopen("ordine.in","r",stdin);
	freopen("ordine.out","w",stdout);
	cit();
	solve();
	return 0;
}