#include <iostream>
#include <fstream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <cstring>
#include <string>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <algorithm>
using namespace std;
 
#define REP(i,n) for(int i=0; i<n; ++i)
#define FOR(i,a,b) for(int i=a; i<=b; ++i)
#define FORR(i,a,b) for (int i=a; i>=b; --i)
#define pi acos(-1.0)
 
typedef long long ll;
typedef vector<int> VI;
typedef vector<ll> VL;
typedef vector<VI> VVI;
typedef pair<int,int> P;
typedef pair<ll,ll> PL;

int main(){
	int n;
	while (cin >> n && n){
		bool f = 1;
		vector<string> s(n);
		REP(i,n) cin >> s[i];
		VVI e(26,VI(26));
		REP(i,n-1){
			string a = s[i], b = s[i+1];
			REP(j,min(a.length(), b.length())){
				if (a[j] != b[j]){
					e[a[j]-'a'][b[j]-'a'] = 1;
					break;
				}
				if (j == min(a.length(), b.length())-1 && a.length() > b.length())
					f = 0;
			}
		}
		REP(i,26){
			VI visit(26);
			queue<int> que;
			que.push(i);
			while (!que.empty()){
				int x = que.front();
				que.pop();
				REP(j,26){
					if (!e[x][j] || visit[j]) continue;
					visit[j] = 1;
					que.push(j);
				}
			}
			if (visit[i]) f = 0;
		}
		cout << (f ? "yes" : "no") << endl;
	}
    return 0;
}