Problem Statement

We found a dictionary of the Ancient Civilization Mayo (ACM) during excavation of the ruins. After analysis of the dictionary, we revealed they used a language that had not more than 26 letters. So one of us mapped each letter to a different English alphabet and typed all the words in the dictionary into a computer.

How the words are ordered in the dictionary, especially whether they are ordered lexicographically, is an interesting topic to many people. As a good programmer, you are requested to write a program to judge whether we can consider the words to be sorted in a lexicographical order.

Note: In a lexicographical order, a word always precedes other words it is a prefix of. For example, `ab` precedes `abc`, `abde`, and so on.



Input

The input consists of multiple datasets. Each dataset is formatted as follows:


n
string_1
...
string_n


Each dataset consists of n+1 lines. The first line of each dataset contains an integer that indicates n (1 \leq n \leq 500). The i-th line of the following n lines contains string_i, which consists of up to 10 English lowercase letters.

The end of the input is `0`, and this should not be processed.

Output

Print either `yes` or `no` in a line for each dataset, in the order of the input. If all words in the dataset can be considered to be ordered lexicographically, print `yes`. Otherwise, print `no`.

Example

Input

4
cba
cab
b
a
3
bca
ab
a
5
abc
acb
b
c
c
5
abc
acb
c
b
b
0


Output

yes
no
yes
no
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
const int inf = 1e9;
const int AtoZ = 26;

bool check(int n, int b, int e, vector<string> &str, vector<vector<int> > &ord){
	char now=str[b][n];
	int s=b;
	for(int i=b; i<=e; i++){
		if(str[i][n] != now){
			if(str[i][n]=='0') return false;
			if(now!='0'){
				ord[now-'a'][str[i][n]-'a'] = 0;
				if(!check(n+1, s, i-1, str, ord)) return false;
			}
			s = i;
			now = str[i][n];
		}
	}
	if(now!='0'){
		if(!check(n+1, s, e, str, ord)) return false;
	}
	return true;
}

int main(){
	while(1){
		int n;
		cin >> n;
		if(n==0) break;
		
		vector<string> str(n);
		vector<vector<int> > ord(AtoZ, vector<int>(AtoZ, inf));
		for(int i=0; i<n; i++){
			cin >> str[i];
			str[i] += "0";
		}
		if(!check(0, 0, n-1, str, ord)){
			cout << "no" << endl;
			continue;
		}
		
		for(int k=0; k<AtoZ; k++){
			for(int i=0; i<AtoZ; i++){
				for(int j=0; j<AtoZ; j++){
					ord[i][j] = min(ord[i][j], ord[i][k]+ord[k][j]);
				}
			}
		}
		bool flag = true;
		for(int i=0; i<AtoZ; i++){
			for(int j=i+1; j<AtoZ; j++){
				if(ord[i][j]==0 && ord[j][i]==0){
					flag = false;
					break;
				}
			}
		}
		if(flag){
			cout << "yes" << endl;
		}else{
			cout << "no" << endl;
		}
	}
    return 0;
}