#include <algorithm>
#include <cmath>
#include <climits>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <cassert>
#include <functional>

using namespace std;

#define LOG(...) printf(__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((c).begin(),(c).end())
#define RSORT(c) sort((c).rbegin(),(c).rend())
#define CLR(a) memset((a), 0 ,sizeof(a))

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;

const int dx[] = { -1, 0, 1, 0 }; const int dy[] = { 0, 1, 0, -1 };

struct UnionFind {
	vector<int> v;
	UnionFind(int n) : v(n)  { for (int i = 0; i < n; i++) v[i] = i; }
	int  find(int x)         { return v[x] == x ? x : v[x] = find(v[x]); }
	void unite(int x, int y) { v[find(x)] = find(y); }
};


int main() {
	int n;
	while (cin >> n, n) {
		vector<string> vs(n);
		vector<set<int>> alp(26);
		string ans = "yes";
		REP(i, n)
			cin >> vs[i];
		REP(i, n-1) {
			int si1=vs[i].length();
			int si2 = vs[i+1].length();
			REP (j,min(si1,si2)) {
				if (vs[i][j] == vs[i + 1][j])
					continue;
				if (alp[vs[i + 1][j] - 'a'].find(vs[i][j] - 'a') != alp[vs[i + 1][j] - 'a'].end()) {
					ans = "no";
					goto res;
				}
				alp[vs[i][j] - 'a'].insert(vs[i + 1][j]-'a');
				REP(k, 26) {
					if (alp[k].find(vs[i][j] - 'a') != alp[k].end()) {
						if (alp[vs[i + 1][j] - 'a'].find(k) != alp[vs[i + 1][j] - 'a'].end()) {
							ans = "no";
							goto res;
						}
						alp[k].insert(vs[i + 1][j] - 'a');
					}
				}
				break;
			}
			if (si1 > si2&&vs[i].substr(0, vs[i + 1].length()) == vs[i + 1]) {
				ans = "no";
				goto res;
			}
		}
	res:
		cout << ans << endl;
	}
}