#include<bits/stdc++.h>
using namespace std;

struct node{
	int count;
	char ch;
};

bool cmp(node A,node B){
	if(A.count!=B.count)return A.count>B.count;
	if(A.count==B.count){
		return A.ch<B.ch;
	}
	
}

node arr[10000];
main(){
 	string str;
 	int count[100000]={0};
 	cin>>str;
 	int n=str.size();
 	for(int i=0;i<n;++i){
 		int j=i+1;
 		if(str[i]==str[j]){
 			int t=str[i]-97;
 			arr[t].ch=str[i];
 			arr[t].count++;
		 }
	 }
	 sort(arr,arr+26,cmp);
	 for(int i=0;i<26;++i){
	 	if(arr[i].count>0)cout<<arr[i].ch<<arr[i].ch<<endl;
	 }
	
	
 }