#include <iostream>
#include <ranges>
#include <algorithm>
#include <vector>
using namespace std;
using ll=long long;

#include <atcoder/all>
using mint=atcoder::modint998244353;

ostream& operator<<(ostream& os,mint& x){
	os<<x.val();
	return os;
}
istream& operator>>(istream& is,mint& x){
	int t;
	is>>t;
	x=t;
	return is;
}

template <typename S,typename T>
ostream& operator<<(ostream& os,const pair<S,T>& p);
template <typename S,typename T>
istream& operator>>(istream& is,pair<S,T>& p);
template <typename T,size_t n>
ostream& operator<<(ostream& os,const array<T,n>& arr);
template <typename T,size_t n>
istream& operator>>(istream& is,array<T,n>& arr);
template <typename T>
ostream& operator<<(ostream& os,const vector<T>& vec);
template <typename T>
istream& operator>>(istream& is,vector<T>& vec);

template <typename S,typename T>
ostream& operator<<(ostream& os,pair<S,T>& p){
	os<<p.first<<" "<<p.second;
	return os;
}
template <typename S,typename T>
istream& operator>>(istream& is,pair<S,T>& p){
	is>>p.first>>p.second;
	return is;
}

template <typename T,size_t n>
ostream& operator<<(ostream& os,array<T,n>& arr){
	for(int i=0;i<n;i++)os<<arr[i]<<(i+1==n?"":" ");
	return os;
}
template <typename T,size_t n>
istream& operator>>(istream& is,array<T,n>& arr){
	for(int i=0;i<n;i++)is>>arr[i];
	return is;
}
template <typename T>
ostream& operator<<(ostream& os,vector<T>& vec){
	for(int i=0;i<vec.size();i++)os<<vec[i]<<(i+1==vec.size()?"":" ");
	return os;
}
template <typename T>
istream& operator>>(istream& is,vector<T>& vec){
	for(int i=0;i<vec.size();i++)is>>vec[i];
	return is;
}

int main(){
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	int n,m;
	cin>>n>>m;
	vector<string> s(n);
	for(int i=0;i<n;i++)cin>>s[i];
	array<int,26> e;
	for(int i=0;i<26;i++)e[i]=-1;
	vector<array<int,26>> trie(1,e);
	vector<int> cnt(1,0);
	vector<int> cnt2(1,0);
	for(int i=0;i<n;i++){
		auto dfs=[&](auto dfs,int v,int d)->void{
			if(d==s[i].size()){
				cnt2[v]++;
				cnt[v]++;
				return;
			}
			int nxt=s[i][d]-'a';
			if(trie[v][nxt]==-1){
				trie[v][nxt]=trie.size();
				cnt.push_back(0);
				cnt2.push_back(0);
				trie.push_back(e);
			}
			cnt[v]-=cnt[trie[v][nxt]];
			dfs(dfs,trie[v][nxt],d+1);
			cnt[v]+=cnt[trie[v][nxt]];
		};
		dfs(dfs,0,0);
	}
	string cur="";
	int sum=0;
	auto dfs=[&](auto dfs,int v,int d)->bool{
		if(sum+cnt[v]+m<=n){
			return true;
		}
		for(int i=0;i<26;i++){
			if(trie[v][i]==-1){
				if(sum+cnt2[v]+m<=n){
					cur+=('a'+i);
					return true;
				}
			}else{
				sum+=cnt2[v];
				cur+=('a'+i);
				if(dfs(dfs,trie[v][i],d+1))return true;
				else{
					cur.pop_back();
					sum-=cnt2[v];
				}
			}
		}
		return false;
	};
	if(dfs(dfs,0,0))cout<<"Yes"<<endl<<cur<<endl;
	else cout<<"No"<<endl;
}