#include "iostream"
#include "climits"
#include "list"
#include "queue"
#include "stack"
#include "set"
#include "functional"
#include "algorithm"
#include "string"
#include "map"
#include "unordered_map"
#include "unordered_set"
#include "iomanip"
#include "cmath"
#include "random"
#include "bitset"
#include "cstdio"
#include "numeric"
#include "cassert"
#include "ctime"

using namespace std;

//constexpr long long int MOD = 1000000007;
//constexpr int MOD = 1000000007;
//constexpr int MOD = 998244353;
constexpr long long int MOD = 998244353;
constexpr double EPS = 1e-9;

//int N, M, K, H, W, L, R;
long long int N, M, K, H, W, L, R;


int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);

	cin >> N;
	while (N) {
		vector<string>s(N);
		for (auto &i : s) {
			cin >> i;
			while (i.size() < 10) {
				i.push_back('z' + 1);
			}
		}
		vector<vector<int>>edge(27, vector<int>(27));
		for (int i = 0; i < 26; i++)edge[26][i] = 1;
		for (int i = 1; i < N; i++) {
			for (int j = 0; j < min(s[i - 1].size(), s[i].size()); j++) {
				if (s[i][j] != s[i - 1][j]) {
					edge[s[i - 1][j] - 'a'][s[i][j] - 'a'] = 1;
					break;
				}
			}
		}
		bool flag = true;
		for (int i = 0; i < 27; i++) {
			vector<int>to(27);
			queue<int>Q;
			Q.push(i);
			while (!Q.empty()) {
				int cn = Q.front();
				Q.pop();
				for (int j = 0; j < 27; j++) {
					if (edge[cn][j]) {
						if (!to[j]) {
							to[j] = 1;
							Q.push(j);
						}
					}
				}
			}
			if (to[i])flag = false;
		}
		if (flag)cout << "yes\n";
		else cout << "no\n";
		cin >> N;
	}

}
