#define _GLIBCXX_DEBUG
#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cmath>
#include<cctype>
#include<math.h>
#include<string>
#include<string.h>
#include<stack>
#include<queue>
#include<vector>
#include<utility>
#include<set>
#include<map>
#include<stdlib.h>
#include<iomanip>
#include<deque>

using namespace std;

#define ll long long
#define ld long double
#define EPS 0.0000000001
#define INF 1e9
#define LINF (ll)INF*INF
#define MOD 1000000007
#define rep(i,n) for(int i=0;i<(n);i++)
#define loop(i,a,n) for(int i=a;i<(n);i++)
#define all(in) in.begin(),in.end()
#define shosu(x) fixed<<setprecision(x)

#define int ll //!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

typedef vector<int> vi;
typedef vector<string> vs;
typedef pair<int,int> pii;
typedef vector<pii> vp;
typedef deque<int> di;

int gcd(int a, int b){
    if(b==0) return a;
    return gcd(b,a%b);
}
int lcm(int a, int b){
    return a*b/gcd(a,b);
}

signed main(void) {
    int n;
    while(cin >> n, n){
        vs s(n);
        rep(i,n)cin >> s[i];
        vector<vi> g(26);
        bool ans = true;
        rep(i,n-1){
            int j = 0;
            while(j < s[i].size() && j < s[i+1].size() && s[i][j] == s[i+1][j])j++;
            if(j == s[i+1].size() && j < s[i].size()){
                ans = false;
                break;
            }
            if(j == s[i].size() || j == s[i+1].size())continue;
            //cout << s[i][j] << " " << s[i+1][j] << endl;
            g[s[i][j]-'a'].push_back(s[i+1][j]-'a');
        }
        rep(i,26){
            queue<int> q;
            rep(j,g[i].size())q.push(g[i][j]);
            vi used(26);
            while(q.size() && ans){
                int v = q.front();
                q.pop();
                used[v]++;
                rep(j,g[v].size()){
                    int ne = g[v][j];
                    if(ne == i){
                        ans = false;
                    }
                    if(used[ne] == 0){
                        q.push(ne);
                    }
                }
            }
        }
        if(ans)cout << "yes" << endl;
        else cout << "no" << endl;
    }
}

