#include <bits/stdc++.h>
using namespace std;
int n;
map<char, char> id;
string s[555];
char idlast;
int main() {
  cin.sync_with_stdio(false);
  while (1) {
    cin >> n;
    if (n == 0) break;
    idlast = 'a';
    id.clear();
    for (int i = (int)(0); i < (int)(n); i++) {
      cin >> s[i];
      for (int j = (int)(0); j < (int)(s[i].length()); j++) {
        if (!id.count(s[i][j])) id[s[i][j]] = idlast++;
      }
    }
    string ss[555];
    for (int i = (int)(0); i < (int)(n); i++)
      for (int j = (int)(0); j < (int)(s[i].length()); j++) {
        ss[i] += id[s[i][j]];
      }
    bool ok = true;
    for (int i = (int)(0); i < (int)(n - 1); i++) {
      if (ss[i] > ss[i + 1]) ok = false;
    }
    if (ok)
      cout << "yes" << endl;
    else
      cout << "no" << endl;
  }
  return 0;
}
