#include <bits/stdc++.h>

using namespace std;

#define LOG(...) fprintf(stderr,__VA_ARGS__)
//#define LOG(...)
#define FOR(i,a,b) for(int i=(int)(a);i<(int)(b);++i)
#define REP(i,n) for(int i=0;i<(int)(n);++i)
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort(ALL(c))
#define RSORT(c) sort(RALL(c))

typedef long long ll;
typedef unsigned long long ull;
typedef vector<bool> vb;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vb> vvb;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;

int main() {
    int n;
    while (cin >> n, n) {
        vector<string> s(n);
        REP(i, n) cin >> s[i];

        bool t[26][26] = {};

        REP(i, n-1) {
            string s1 = s[i];
            string s2 = s[i+1];
            REP(j, s1.length()) {
                if ((int)s2.length() <= j) {
                    cout << "no" << endl;
                    goto NEXT;
                }
                if (s1[j] != s2[j]) {
//                    LOG("--> %c < %c\n", s1[j], s2[j]);
                    t[s1[j]-'a'][s2[j]-'a'] = true;
                    break;
                }
            }
        }

        REP(i, 26) REP(j, 26) REP(k, 26) t[j][k] |= t[j][i] && t[i][k];

        // ???????????????????????°no
        REP(i, 26) {
            if (t[i][i]) {
                cout << "no" << endl;
                goto NEXT;
            }
        }
        cout << "yes" << endl;
NEXT: {}
    }
}