#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic warning "-Wunused-function"
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

struct Node {
  int cnt = 0, acc = 0;
  Node* ch[26]{};
  ~Node() {
    rep(i, 26) if (ch[i]) delete ch[i];
  }
};

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, m;
  cin >> n >> m;
  Node head;
  rep(_, n) {
    string s;
    cin >> s;
    Node* p = &head;
    static vector<Node*> hist;
    hist = {p};
    for (char c : s) {
      int x = c - 'a';
      if (!p->ch[x]) p->ch[x] = new Node;
      p = p->ch[x];
      hist.emplace_back(p);
    }
    p->cnt++;
    for (auto p : hist) p->acc++;
  }
  string ans;
  bool found = false;
  auto dfs = [&](auto&& self, Node* p, int seen) -> void {
    int now = n - seen;
    if (p) now -= p->acc;
    if (now >= m) {
      found = true;
      return;
    }
    if (!p) return;
    seen += p->cnt;
    rep(x, 26) {
      ans += 'a' + x;
      self(self, p->ch[x], seen);
      if (found) return;
      ans.pop_back();
    }
  };
  dfs(dfs, &head, 0);
  if (!found) {
    cout << "No\n";
  } else {
    cout << "Yes\n";
    cout << ans << '\n';
  }
}