#include <bits/stdc++.h>
#define N 1000011
#define ull unsigned long long
using namespace std;
int t,n,nxt[N];ull pw[N];char s[N];
struct str{ull H;int len,hd,tl;}a[N];
str operator+(str a,str b)
{
	str c;
	c.len=a.len+b.len;
	c.H=a.H*pw[b.len]+b.H;
	c.hd=a.hd;c.tl=b.tl;
	nxt[a.tl]=b.hd;
	return c;
}
const int B=801;
void print(int U)
{
	while(~U)putchar(s[U]),U=nxt[U];
}
int main()
{
	scanf("%s",s+1);n=strlen(s+1);
	for(int i=0;i<=n;++i)nxt[i]=-1;
	sort(s+1,s+1+n);
	pw[0]=1;for(int i=1;i<=n;++i)pw[i]=pw[i-1]*B;
	for(int i=1;i<=n;++i)a[i].H=s[i],a[i].len=1,a[i].hd=a[i].tl=i;
	int pt=n;
	while(pt>1)
	{
		int k=n;
		for(int i=1;i<=n;++i)if(a[i].H!=a[1].H){k=i-1;break;}
		while(k)
		{
			a[k]=a[k]+a[pt];
			--pt;--k;
		}
	}
	int U=a[1].hd;
	while(~U)putchar(s[U]),U=nxt[U];putchar(10);return 0;
}