#include <bits/stdc++.h>
using namespace std;
map<string, int> mp;
signed main(){
	int n, f;
	cin >> n >> f;
	string s;
	cin >> s;
	set<string> st;
	for(int i = 0; i < s.length() - 2; i++){
		string s1 = s.substr(i, 3);
		if(s1[1] == s1[2]){
			mp[s1]++;
			for(int j = 0; j < 26; j++){
				s1[0] = char('a' + j);
				if(mp[s1] + 1 >= f){
					st.insert(s1);
				}
			}
		   i += 3;
		   if(i >= s.length()) break; 
		}
 
	}
	for(string i: st){
		cout << i << endl; 
	}
 
}

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MTcgMgptb21vb2JhYWFhYXFxcWNxcQ==

17 2
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