#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  string str;
  while (cin >> n, n) {
    map<char, char> bef, aft;
    cin >> str;
    for (int i = 0; i < (int)str.size(); i++) {
      bef[str[i]] = i + 'a';
      aft[i + 'a'] = str[i];
    }
    string s[500], t[500];
    for (int i = 0; i < n - 1; i++) {
      cin >> s[i];
      string change;
      for (int j = 0; j < (int)s[i].size(); j++) {
        change += bef[s[i][j]];
      }
      t[i] = change;
    }
    bool flg = true;
    sort(t, t + n - 1);
    for (int i = 0; i < n - 1; i++) {
      string change;
      for (int j = 0; j < (int)t[i].size(); j++) {
        change += aft[t[i][j]];
      }
      if (change != s[i]) {
        flg = false;
        break;
      }
    }
    if (flg) {
      cout << "yes" << endl;
    } else {
      cout << "no" << endl;
    }
  }
  return 0;
}
