#include <bits/stdc++.h>
#define rep(i, a, n) for(int i = a; i < n; i++)
#define int long long
using namespace std;
typedef pair<int, int> P;

bool G[26][26];
bool f[26];
bool ans;

void dfs(int now, int root){
    // cout << now << endl;
    if(f[now]){
        if(now == root) ans = false;
        return;
    }
    f[now] = true;
    rep(i, 0, 26){
        if(G[now][i]) dfs(i, root);
    }
}

signed main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n;
    while(cin >> n, n){
        vector<string> s(n);
        ans = true;
        rep(i, 0, 26) rep(j, 0, 26) G[i][j] = false;
        rep(i, 0, n){
            cin >> s[i];
        } 
        rep(i, 0, n - 1){
            int lm = min(s[i].size(), s[i + 1].size());
            bool same = true;
            rep(j, 0, lm){
                if(s[i][j] == s[i + 1][j]) continue;
                else{
                    same = false;
                    G[s[i][j] - 'a'][s[i + 1][j] - 'a'] = true;
                    break;
                }
            }
            if(same){
                if(s[i].size() > s[i + 1].size()){
                    ans = false;
                    break;
                }
            }
        }
        rep(i, 0, 26){
            memset(f, false, sizeof(f));
            dfs(i, i);
        }
        if(ans) cout << "yes" << endl;
        else cout << "no" << endl;
    }
}
