/* -*- coding: utf-8 -*-
 *
 * 3626.cc:  No.3626 Not a Prefix - yukicoder
 */

#include<cstdio>
#include<algorithm>

using namespace std;

/* constant */

const int MAX_N = 200000;
const int MAX_GN = 500000;
const int INF = 1 << 30;

/* typedef */

struct Node {
  int p, c, f, nbrs[26];
  Node(): p(), c(), f(), nbrs() {}

  void init(int _p = -1) {
    p = _p, c = f = 0;
    fill(nbrs, nbrs + 26, -1);
  }
};

/* global variables */

char s[MAX_GN + 4];
Node es[MAX_GN];
int cis[MAX_GN], mincs[MAX_GN];

/* subroutines */

int allocnode(int &gn, int p = -1) {
  es[gn].init(p);
  return gn++;
}

void trie_add(char s[], int &gn) {
  int u = 0;
  for (int i = 0; s[i]; i++) {
    int ci = s[i] - 'a';
    int &v = es[u].nbrs[ci];
    if (v < 0) v = allocnode(gn, u);
    es[v].c++;
    u = v;
  }
  es[u].f++;
}

/* main */

int main() {
  int n, m;
  scanf("%d%d", &n, &m);

  int gn = 0;
  allocnode(gn);
  
  for (int i = 0; i < n; i++) {
    scanf("%s", s);
    trie_add(s, gn);
  }

  fill(mincs, mincs + gn, INF);
  for (int u = 0; u >= 0;) {
    auto &nbru = es[u].nbrs;
    int up = es[u].p;

    if (cis[u] < 26) {
      int v = nbru[cis[u]++];
      if (v >= 0) u = v;
    }
    else {
      mincs[u] = min(mincs[u], es[u].c);
      if (up >= 0) mincs[up] = min(mincs[up], mincs[u]);
      u = up;
    }
  }

  int l = 0, d = n - m;
  for (int u = 0;;) {
    auto &nbru = es[u].nbrs;
    bool cont = false, f = false;
    for (int i = 0; i < 26; i++) {
      int v = nbru[i];
      if (v < 0 || (es[v].f <= d && mincs[v] <= d)) {
	s[l++] = 'a' + i;
	cont = true;
	if (es[v].c <= d) f = true;
	u = v;
	break;
      }
    }
    if (! cont || f) break;
  }
  s[l] = '\0';

  if (l > 0) printf("Yes\n%s\n", s);
  else puts("No");
  
  return 0;
}