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

using ll = int64_t;
using ul = uint64_t;
using ld = long double;
using vi = vector<int>;
using vd = vector<double>;
using vc = vector<char>;
using vs = vector<string>;
using vb = vector<bool>;
using vl = vector<ll>;
using vul = vector<ul>;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
using vvi = vector<vi>;
using vvd = vector<vd>;
using vvc = vector<vc>;
using vvb = vector<vb>;
using vvl = vector<vl>;
using mint = modint998244353;
using vm = vector<mint>;

template<int char_siz, char base>
struct Trie {
  // 各頂点の情報を持つ型
  struct Node {
    vi next, end;
    int c, comm = 0;
    Node(int c_) : c(c_) {
      next.assign(char_siz, -1);
    }
  };
  
  // tree
  vector<Node> data;
  int cnt;
  Trie() : cnt(1) { data.push_back(Node(0)); }

  // insert
  void insert(string &S, int id) {
    int now = 0, siz = S.size();
    for (int i = 0; i < siz; i++) {
      data[now].comm++;
      int c = S[i] - base;
      if (data[now].next[c] == -1) {
        data[now].next[c] = cnt;
        data.push_back(Node(c));
        cnt++;
      }
      now = data[now].next[c];
    }
    data[now].end.push_back(id);
    data[now].comm++;
    return;
  }

  bool search(int rem, stack<char> &hist, int now) {
    if (data[now].comm <= rem && hist.size() > 0) {
      string ans = "";
      while(!hist.empty()) {
        ans += hist.top();
        hist.pop();
      }
      reverse(ans.begin(),ans.end());
      cout << "Yes" << endl << ans << endl;
      return true;
    }
    rem -= data[now].end.size();
    if (rem <= 0) return false;
    for (int i = 0; i < char_siz; i++) {
      hist.push(base + i);
      if (search(rem, hist, data[now].next[i])) return true;
      hist.pop();
    }
    return false;
  }
};

int main() {
  int N,M;
  cin >> N >> M;
  Trie<26, 'a'> trie;
  for (int i = 0; i < N; i++) {
    string S;
    cin >> S;
    trie.insert(S, i);
  }
  stack<char> hist;
  if (!trie.search(N - M, hist, 0)) cout << "No" << endl;
  return 0;
}