10659번 - Dictionary 다국어

문제

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.

입력

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

n
string1
...
stringn

Each dataset consists of n+1 lines. The first line of each dataset contains an integer that indicates n (1 ≤ n ≤ 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.

출력

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.

제한

힌트