#include <bits/stdc++.h>
using namespace std;
struct SCC {
  int V;
  vector<int> G[100000];
  vector<int> rG[100000];
  vector<int> vs;
  bool used[100000];
  int cmp[100000];
  SCC() {
    fill_n(used, 100000, 0);
    fill_n(cmp, 100000, 0);
    V = 33;
  }
  void add_edge(int from, int to) {
    G[from].push_back(to);
    rG[to].push_back(from);
  }
  void dfs(int v) {
    used[v] = true;
    for (int i = 0; i < G[v].size(); i++)
      if (!used[G[v][i]]) dfs(G[v][i]);
    vs.push_back(v);
  }
  void rdfs(int v, int k) {
    used[v] = true;
    cmp[v] = k;
    for (int i = 0; i < rG[v].size(); i++)
      if (!used[rG[v][i]]) rdfs(rG[v][i], k);
  }
  int scc() {
    memset(used, 0, sizeof(used));
    vs.clear();
    for (int v = 0; v < V; v++)
      if (!used[v]) dfs(v);
    memset(used, 0, sizeof(used));
    int k = 0;
    for (int i = vs.size() - 1; i >= 0; i--) {
      if (!used[vs[i]]) rdfs(vs[i], k++);
    }
    return k;
  }
};
int main() {
  int n;
  while (cin >> n, n) {
    string s[555];
    int use[33] = {};
    int cnt = 0;
    int ans = 1;
    SCC sc;
    for (int i = 0; i < n; i++) cin >> s[i];
    int ma[33][33] = {};
    for (int i = 0; i < n; i++) {
      for (int j = i + 1; j < n; j++) {
        string a = s[i], b = s[j];
        int l = min((int)a.size(), (int)b.size());
        if (a == b) continue;
        int flg = 1;
        for (int k = 0; k < l; k++) {
          if (a[k] == b[k]) continue;
          flg = 0;
          sc.add_edge(a[k] - 'a', b[k] - 'a');
          if (use[a[k] - 'a'] == 0) {
            use[a[k] - 'a'] = 1;
            cnt++;
          }
          if (use[b[k] - 'a'] == 0) {
            use[b[k] - 'a'] = 1;
            cnt++;
          }
          break;
        }
        if (flg) ans = 0;
      }
    }
    int tmp = sc.scc();
    if (tmp == 33 && ans)
      cout << "yes" << endl;
    else
      cout << "no" << endl;
  }
  return 0;
}
