#include<bits/extc++.h>
#include<bits/stdc++.h>
//#pragma GCC optimize(2)
//本代码含有小众情感元素，建议满 18 周岁后观看
#ifdef __unix__
#define getchar getchar_unlocked
#define putchar putchar_unlocked
#else
#define getchar _getchar_nolock
#define putchar _putchar_nolock
#endif
#define debug fprintf(stderr,"Tomoyo after ~It's a wonderful life!~\n")
#define all(x) x.begin(),x.end()
#define pii pair<int,int>
#define Inf (int)INFINITY
#define inf 0x3f3f3f3f
#define pb push_back
#define ll long long
#define endl '\n'
#define y second
#define x first
using namespace std;
const int N=1e6+10;
void read(){};
template<class T1,class ...T2>
inline void read(T1& ret,T2&... rest){
	ret=0;char c=getchar();bool f=false;
	while(c<'0'||c>'9')f=c=='-',c=getchar();
	while(c>='0'&&c<='9')ret=ret*10+c-'0',c=getchar();
	f&&(ret=-ret),read(rest...);
}
inline void read(string&ret){
	ret="";char c=getchar();
	while(isspace(c))c=getchar();
	while(!isspace(c))ret+=c,c=getchar();
}
#define cin(...) read(__VA_ARGS__)
template<class T>
inline void print(T x){
	static int s[35],t=0;
	bool f=x<0;if(f)x=-x;
	do s[t++]=x%10,x/=10;while(x);
	f&&putchar('-');while(t)putchar(s[--t]+'0');
}
int n,f;
string s;
map<string,int>mp;
int main(){
	cin(s);
	for(char c:s)mp[string(1,c)]++;
	while(mp.size()>1){
		auto it1=*mp.begin(),it2=*prev(mp.end());
		if(it1.y==it2.y)
			mp.erase(it1.x),mp.erase(it2.x),mp.insert({it1.x+it2.x,it1.y});
		else if(it1.y>it2.y)
			mp.erase(it2.x),mp.insert({it1.x+it2.x,it2.y}),mp[it1.x]-=it2.y;
		else
			mp.erase(it1.x),mp.insert({it1.x+it2.x,it1.y}),mp[it2.x]-=it1.y;
		if(mp.size()==1)break;
		string s1=mp.begin()->x,s2=next(mp.begin())->x;
		for(int i=0;i<(int)s1.size()&&i<(int)s2.size()&&!f;i++)f=(s1[i]!=s2[i]);
		if(f){
			for(auto it=prev(mp.end());it!=mp.begin();it=prev(mp.end())){
				for(;;){
					auto it1=*mp.begin(),it2=*it;
					if(it1.y==it2.y){
						mp.erase(it1.x),mp.erase(it2.x),mp.insert({it1.x+it2.x,it1.y});
						break;
					}
					else if(it1.y>it2.y){
						mp.erase(it2.x),mp.insert({it1.x+it2.x,it2.y}),mp[it1.x]-=it2.y;
						break;
					}
					else
						mp.erase(it1.x),mp.insert({it1.x+it2.x,it1.y}),mp[it2.x]-=it1.y;
				}
			}
			break;
		}
	}
	cout<<(mp.begin()->x)<<endl;
	return 0;
}