e-olymp
Competitions

ADA Classes - November 20

Dictionary

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

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 (1n500). 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".

Time limit 1 second
Memory limit 128 MiB
Input example #1
4
cba
cab
b
a
3
bca
ab
a
5
abc
acb
b
c
c
5
abc
acb
c
b
b
0
Output example #1
yes
no
yes
no
Source 2012 Japanese Alumni Group Practice Contest Asia Regional, AtCoder, November 4; 2013 Petrozavodsk Winter Training Camp, January 26, Problem C