#include <algorithm>
#include <atcoder/segtree>
#include <bitset>
#include <cassert>
#include <cmath>
#include <ctime>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
using namespace std;
using namespace atcoder;
typedef long long ll;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repr(i, n) for (int i = (int)(n) - 1; i >= 0; i--)
#define repk(i, k, n) for (int i = k; i < (int)(n); i++)
#define all(v) v.begin(), v.end()
#define mod1 1000000007
#define mod2 998244353
#define mod3 100000007
#define vi vector<int>
#define vs vector<string>
#define vc vector<char>
#define vl vector<ll>
#define vb vector<bool>
#define vvi vector<vector<int>>
#define vvc vector<vector<char>>
#define vvl vector<vector<ll>>
#define vvb vector<vector<bool>>
#define vvvi vector<vector<vector<int>>>
#define vvvl vector<vector<vector<ll>>>
#define pii pair<int, int>
#define pil pair<int, ll>
#define pli pair<ll, int>
#define pll pair<ll, ll>
#define vpii vector<pair<int, int>>
#define vpll vector<pair<ll, ll>>
#define vvpii vector<vector<pair<int, int>>>
#define vvpll vector<vector<pair<ll, ll>>>

template <typename T>
void debug(T e) {
    cerr << e << endl;
}

template <typename T>
void debug(vector<T> &v) {
    rep(i, v.size()) { cerr << v[i] << " "; }
    cerr << endl;
}

template <typename T>
void debug(vector<vector<T>> &v) {
    rep(i, v.size()) {
        rep(j, v[i].size()) { cerr << v[i][j] << " "; }
        cerr << endl;
    }
}

template <typename T>
void debug(vector<pair<T, T>> &v) {
    rep(i, v.size()) { cerr << v[i].first << " " << v[i].second << endl; }
}

template <typename T>
void debug(set<T> &st) {
    for (auto itr = st.begin(); itr != st.end(); itr++) {
        cerr << *itr << " ";
    }
    cerr << endl;
}

template <typename T>
void debug(multiset<T> &ms) {
    for (auto itr = ms.begin(); itr != ms.end(); itr++) {
        cerr << *itr << " ";
    }
    cerr << endl;
}

template <typename T>
void debug(map<T, T> &mp) {
    for (auto itr = mp.begin(); itr != mp.end(); itr++) {
        cerr << itr->first << " " << itr->second << endl;
    }
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
    cerr << H << " ";
    debug_out(T...);
}

bool done = false;

void dfs(string &now_str, vector<vector<ll>> &trie, ll v, ll rem, vector<ll> &cnt, vector<bool> &ends){
    if (done) return;
    if (ends[v]) return;
    ll sum = 0;
    for (ll i = 0; i < 26; i++){
        if (trie[v][i] != -1){
            sum += cnt[trie[v][i]];
        }
    }
    for (ll i = 0; i < 26; i++){
        if (done) return;
        now_str += (char)(i + 'a');
        if (trie[v][i] == -1){
            done = true;
            cout << "Yes" << endl;
            cout << now_str << endl;
            return;
        }
        else if (sum - cnt[trie[v][i]] >= rem){
            done = true;
            cout << "Yes" << endl;
            cout << now_str << endl;
            return;
        }
        else{
            dfs(now_str, trie, trie[v][i], rem - (sum - cnt[trie[v][i]]), cnt, ends);
        }
        now_str.pop_back();
    }
    return;
}

int main() {
    ll N, M;
    cin >> N >> M;
    vector<string> S(N);
    rep(i, N) cin >> S[i];

    // Trie 木を用意して、答えを求めていく
    vector<vector<ll>> trie(1, vector<ll>(26, -1));
    vector<ll> cnt(1, 0);
    vector<bool> ends(1, false);
    
    for (ll i = 0; i < N; i++){
        ll len = S[i].size();
        ll now = 0;
        for (ll j = 0; j < len; j++){
            if (trie[now][(ll)(S[i][j] - 'a')] == -1){
                ll len_trie = trie.size();
                vector<ll> vec(26, -1);
                trie.push_back(vec);
                trie[now][(ll)(S[i][j] - 'a')] = len_trie;
                cnt.push_back(0);
                ends.push_back(false);
            }
            now = trie[now][(ll)(S[i][j] - 'a')];
            cnt[now]++;
        }
        ends[now] = true;
    }
    bool pos = true;
    // この trie から答えを構成する方法を考えたい
    // 進んでいって、終端まで行ってしまったら、そこは可能性として有り得ない
    // そこに行く前に逃げ道が見つかれば可能性あり, DFS
    ll now = 0;

    string now_str = "";
    dfs(now_str, trie, 0, M, cnt, ends);
    if (!done) cout << "No" << endl;
}