#include <bits/stdc++.h>

using namespace std;

const int M = 26;

char g[M + 1][M + 1]; // g[i][j] char[i] > char[j]
int visit[M + 1];

bool dfs(int i) {
	visit[i] = 1;
	for(int j = 0;j <= M;++j){
		if(g[i][j]) {
			if(visit[j] == 2) continue;
			if(visit[j] == 1) return false;
			if(!dfs(j)) return false;
		}
	}
	visit[i] = 2;
	return true;
}

void ordering(string &a, string &b){
	int len = max(a.length(), b.length());
	for(int i = 0;i < len;++i){
		if(a.length() == i){
			break;
		}
		else if(b.length() == i){
			g[M][a[i] - 'a'] = 1;
			break;
		}
		else if(a[i] != b[i]){
			g[b[i] - 'a'][a[i] - 'a'] = 1;
			break;
		}
	}
	return;
}

int main() {
	int n;
	string s;
	bool judge;
	while(cin >> n, n) {
		vector<string> seq;
		set<char> m;
		memset(g, 0, sizeof(g));
		memset(visit, 0, sizeof(visit));
		judge = true;
		for(int i = 0;i < n;++i){
			cin >> s;
			seq.emplace_back(s);
		}
		g[seq[0][0] - 'a'][M] = 1;
		for(int i = 1;i < n;++i){
			ordering(seq[i - 1], seq[i]);
		}
		for(int i = 0;i <= M;++i){
			judge = judge && dfs(i);
		}
		if(judge){
			cout << "yes" << endl;
		}
		else{
			cout << "no" << endl;
		}
	}
	return 0;
}

