#define  _CRT_SECURE_NO_WARNINGS
#pragma comment (linker, "/STACK:526000000")

#include "bits/stdc++.h"

using namespace std;
typedef string::const_iterator State;
#define eps 1e-11L
#define MAX_MOD 1000000007LL
#define GYAKU 500000004LL

#define MOD 998244353LL
#define seg_size 262144 * 4LL
#define pb push_back
#define mp make_pair
typedef long long ll;
#define REP(a,b) for(long long (a) = 0;(a) < (b);++(a))
#define ALL(x) (x).begin(),(x).end()

unsigned long xor128() {
	static unsigned long x = 123456789, y = 362436069, z = 521288629, w = time(NULL);
	unsigned long t = (x ^ (x << 11));
	x = y; y = z; z = w;
	return (w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)));
}
void init() {
	iostream::sync_with_stdio(false);
	cout << fixed << setprecision(20);
}

#define int ll

void solve(){
	while (true){
		starter:;
		int n;
		cin >> n;
		if (n == 0) return;
		int dist[27][27] = {};
		vector<string> inputs;
		REP(i, n) {
			string s;
			cin >> s;
			inputs.push_back(s);
		}
		for (int i = 1; i < inputs.size(); ++i) {
			int ok = 0;
			REP(q, min(inputs[i-1].size(),inputs[i].size())) {
				if (inputs[i][q] != inputs[i - 1][q]) {
					dist[inputs[i - 1][q] - 'a'][inputs[i][q] - 'a'] = 1;
					ok = 1;
					break;
				}
			}
			if (ok == 0) {
				if (inputs[i - 1].length() > inputs[i].length()) {
					cout << "no" << endl;
					goto starter;
				}
			}
		}
		REP(t, 3) {
			REP(i, 26) {
				REP(q, 26) {
					REP(j, 26) {
						if(dist[i][j] != 0&&dist[j][q] != 0)
						dist[i][q] = max(dist[i][q], dist[i][j] + dist[j][q]);
					}
				}
			}
		}
		int copy_dist[27][27];
		REP(i, 27) {
			REP(q, 27) {
				copy_dist[i][q] = dist[i][q];
			}
		}
		REP(t, 3) {
			REP(i, 26) {
				REP(q, 26) {
					REP(j, 26) {
						if (dist[i][j] != 0 && dist[j][q] != 0)
							dist[i][q] = max(dist[i][q], dist[i][j] + dist[j][q]);
					}
				}
			}
		}
		int ok = 1;
		REP(i, 26) {
			REP(q, 26) {
				if (dist[i][q] != copy_dist[i][q]) ok = 0;
			}
		}
		string ans[2] = { "no","yes" };
		cout << ans[ok] << endl;
	}
}

#undef int

int main() {
	init();
	solve();
}
