#include<iostream>
#include<string>
#include<algorithm>
#include<vector>
#include<iomanip>
#include<math.h>
#include<complex>
#include<queue>
#include<deque>
#include<stack>
#include<map>
#include<set>
#include<bitset>
#include<functional>
#include<assert.h>
#include<numeric>
using namespace std;
#define REP(i,m,n) for(int i=(int)(m) ; i < (int) (n) ; ++i )
#define rep(i,n) REP(i,0,n)
using ll = long long;
const int inf=1e9+7;
const ll longinf=1LL<<60 ;
const ll mod=1e9+7 ;


void solve(int n){
    string s[n];
    rep(i,n)cin>>s[i];
    vector<int> v[26];
    vector<int> cnt(26);
    rep(i,n-1){
        int m=min(s[i].size(),s[i+1].size());
        bool used=0;
        rep(j,m){
            if(s[i][j]!=s[i+1][j]){
                v[s[i][j]-'a'].push_back(s[i+1][j]-'a');
                cnt[s[i+1][j]-'a']++;
                used=true;
                break;
            }
        }
        if(!used&&s[i].size()>m){
            cout<<"no"<<endl;
            return;
        }
    }
    queue<int> q;
    int ans=0;
    rep(i,26){
        if(!cnt[i])++ans,q.push(i);
    }
    while(q.size()){
        int x=q.front();q.pop();
        for(auto to : v[x]){
            if(--cnt[to]==0)++ans,q.push(to);
        }
    }
    if(ans==26)cout<<"yes"<<endl;
    else cout<<"no"<<endl;
}
int main(){
    int n;
    while(cin>>n,n)solve(n);
    return 0;
}
