#include <iostream>
#include <iomanip>
#include <cmath>
#include <cstdio>
#include <algorithm>
#include <vector>
#include <memory>
#include <cstring>
#include <cassert>
#include <numeric>
#include <sstream>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <cctype>
#include <unordered_map>
#include <unordered_set>
using namespace std;

#define REP2(i, m, n) for (int i = (int)(m); i < (int)(n); i++)
#define REP(i, n) REP2(i, 0, n)
#define ALL(S) (S).begin(), (S).end()
typedef long long ll;

int main(){
  ios::sync_with_stdio(false);
  int n;
  while (cin >> n && n){
    vector<string> vs(n);
    REP(i, n) cin >> vs[i];
    
    bool ok = true;
    int g[26][26];
    memset(g, 0, sizeof(g));

    REP(j, n) REP(i, j){
      
      REP(k, min(vs[i].size(), vs[j].size())) {
        if (vs[i][k] != vs[j][k]){
          g[vs[i][k] - 'a'][vs[j][k] - 'a'] = 1;
          break;
        } else if (k + 1 == min(vs[i].size(), vs[j].size()) && vs[i].size() > vs[j].size()){
          ok = false;
        }
      }
    }

    REP(k, 26)REP(i, 26)REP(j, 26){
      g[i][j] = g[i][j] || (g[i][k] * g[k][j]);
    }

    REP(i, 26)REP(j, i){
      if (g[i][j] && g[j][i]) ok = false;
    }
    if (ok){
      cout << "yes" << endl;
    } else {
      cout << "no" << endl;
    }
  }
  return 0;
}