#include<iostream>
#include<string>
#include<iomanip>
#include<cmath>
#include<vector>
#include<algorithm>
#include<queue>

using namespace std;

#define int long long
#define rep(i,n) for(int i = 0; i < (n); i++)
#define INF (long long)(1e18)
#define MOD (int)(1e9+7)

#define yn(f) ((f)?"yes":"no")
#define YN(f) ((f)?"YES":"NO")

#define MAX 510

string s[MAX];
bool temp[30][30];
int ans;

void fuga(int a, int b){//cout<<(char)(a+'a')<<" "<<(char)(b+'a')<<endl;
	if(a==b||!ans) return;
	temp[a][b] = true;
	if(temp[a][b]&&temp[b][a]){
		ans = false;
		//cout<<"XXXX "<<(char)(a+'a')<<" "<<(char)(b+'a')<<endl;
	}
}

void hogehoge(int a, int b){
	if(a==b||!ans)return;
	
	queue<int> Q;
	bool used[30] = {};
	Q.push(a);
	
	while(!Q.empty()){
		int t = Q.front();Q.pop();
		used[t] = true;//cout<<t<<endl;
		fuga(t,b);
		for(int i = 0 ; i < 26; i++){
			if(!used[i]&&temp[i][t]) Q.push(i);
		}
	}
}

void hoge(string a, string b){
	if(!ans) return;
	int flag = 1;
	for(int i = 0,size = min(a.size(),b.size()); i < size; i++){//cout<<i<<"---"<<endl;
		if(flag)hogehoge(a[i]-'a',b[i]-'a');
		if(a[i] != b[i]) flag = false;
	}
	if(flag&&b.size()<a.size())ans = false;
}

signed main(){
	cout<<fixed<<setprecision(7);
	
	
	while(true){
		int n;
		
		ans = true;
		rep(i,30)rep(j,30)temp[i][j]=false;
		
		cin>>n;
		
		if(!n) break;
		
		for(int i = 0; i < n; i++){
			cin>>s[i];
			if(i) hoge(s[i-1],s[i]);
	}
	
		cout<<yn(ans)<<endl;
	}
	
	return 0;
}
