#include<iostream>
#include<cstdio>
#include<algorithm>
#define N 2000000
#define mod1 998244353
#define mod2 998244853
#define Base1 131
#define Base2 171
using namespace std;
int read()
{
	char c=0;
	int sum=0;
	while (c<'0'||c>'9') c=getchar();
	while ('0'<=c&&c<='9') sum=sum*10+c-'0',c=getchar();
	return sum;
}
int MD1(int x)
{
	return x>=mod1?x-mod1:x;
}
int MD2(int x)
{
	return x>=mod2?x-mod2:x;
}
struct reads
{
	int l,r,hsh1,hsh2;
	bool operator == (const reads &t)const
	{
		return hsh1==t.hsh1&&hsh2==t.hsh2;
	}
};
int n,top,l=1,r,nt[N+1];
string s;
reads stk[N+1],dque[N+1];
reads operator + (reads a,reads b)
{
	nt[a.r]=b.l;
	return (reads){a.l,b.r,MD1(1ll*a.hsh1*Base1%mod1+b.hsh1),MD2(1ll*a.hsh2*Base2%mod2+b.hsh2)};
}
int main()
{
	int d,x;
	cin>>s,n=s.length(),sort(s.begin(),s.end());
	for (int i=n;i>=1;--i)
	{
		if (s[i-1]==s[0]) stk[++top]=(reads){i,i,s[i-1],s[i-1]};
		else dque[++r]=(reads){i,i,s[i-1],s[i-1]};
	}
	while (l<=r)
	{
		d=min(top,r-l+1);
	    for (int i=1;i<=d;++i) stk[i]=stk[i]+dque[l++];
		d=top,reverse(stk+1,stk+d+1),top=0;
		for (int i=d;i>=1;--i)
		{
			if (stk[1]==stk[i]) top=max(top,i);
			else dque[++r]=stk[i];
		}
	}
	for (int i=2;i<=top;++i) stk[1]=stk[1]+stk[i];
	x=stk[1].l;
	for (int i=1;i<=n;++i) printf("%c",s[x-1]),x=nt[x];
	puts("");
	return 0;
}