#include <iostream>
#include <vector>
using namespace std;
int INF = 1e9;

bool has_loop(vector< vector<int> > G){
  int n = G.size();
  for(int k = 0; k < n; ++k){
    for(int i = 0; i < n; ++i){
      for(int j = 0; j < n; ++j){
        G[i][j] = min(G[i][j],G[i][k] + G[k][j]);
      }
    }
  }
  for(int i = 0; i < n; ++i){
    if(G[i][i] < INF) return true;
  }
  return false;
}

int main(){
  int n;
  while(cin >> n, n){
    vector<string> S(n);
    for(int i = 0; i < n; ++i){
      cin >> S[i];
    }
    if(n == 1){
      cout << "yes" << endl;
      continue;
    }
    vector< vector<int> > G(26, vector<int>(26,INF));
    bool f = true, g = false;
    for(int i = 1; i < n; ++i){
      if(S[i-1] == S[i]) continue;
      for(int j = 0; j < min(S[i].length(),S[i-1].length()); ++j){
        f = true;
        if(S[i][j] != S[i-1][j]){
          //G[S[i-1][j] - 'a'].push_back(S[i][j] - 'a');
          G[S[i-1][j] - 'a'][S[i][j] - 'a'] = 1;
          f = false;
          break;
        }
      }
      if(f && S[i-1].size() > S[i].size()){
        cout << "no" << endl;
        g = true;
        break;
      }else{
        f = false;
      }
    }
    if(g) continue;

    if(has_loop(G)) cout << "no" << endl;
    else cout << "yes" << endl;
  }
  return 0;
}

