#ifdef NACHIA
#define _GLIBCXX_DEBUG
#else
// disable assert
#define NDEBUG
#endif
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <map>
using namespace std;
using ll = long long;
const ll INF = 1ll << 60;
#define REP(i,n) for(ll i=0; i<ll(n); i++)
template <class T> using V = vector<T>;
template <class A, class B> void chmax(A& l, const B& r){ if(l < r) l = r; }
template <class A, class B> void chmin(A& l, const B& r){ if(r < l) l = r; }


struct Node {
  ll lower = 0;
  ll upper = 0;
  ll just = 0;
  map<ll,ll> nx;
};

void testcase(){
  ll N, M; cin >> N >> M; M = N - M;
  V<string> S(N); REP(i,N) cin >> S[i];
  V<Node> trie(1);
  for(auto& s : S){
    ll p = 0;
    for(char c : s){
      if(!trie[p].nx.count(c)){
        trie[p].nx[c] = trie.size();
        trie.push_back(Node());
      }
      p = trie[p].nx[c];
    }
    trie[p].lower += 1;
    trie[p].upper += 1;
    trie[p].just += 1;
  }

  ll Z = trie.size();
  REP(i,Z) for(auto [u,v] : trie[i].nx) trie[v].upper += trie[i].upper;
  for(ll i=Z-1; i>=0; i--) for(auto [u,v] : trie[i].nx) trie[i].lower += trie[v].lower;

  string ans;

  auto dfs = [&](auto& dfs, ll p) -> bool {
    if(p != 0 && trie[p].lower + trie[p].upper - trie[p].just <= M) return 1;
    if(trie[p].upper > M) return 0;
    for(ll k='a'; k<='z'; k++){
      if(trie[p].nx.count(k)){
        if(dfs(dfs, trie[p].nx[k])){
          ans.push_back(k);
          return 1;
        }
      } else {
        ans.push_back(k);
        return 1;
      }
    }
    return 0;
  };
  if(dfs(dfs, 0)){
    reverse(ans.begin(), ans.end());
    cout << "Yes\n";
    cout << ans << "\n";
  } else {
    cout << "No\n";
  }
}

int main(){
  cin.tie(0)->sync_with_stdio(0);
  testcase();
  return 0;
}