CF16A.Flag

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题目描述

According to a new ISO standard, a flag of every country should have a chequered field $n×m$ , each square should be of one of 10 colours, and the flag should be «striped»: each horizontal row of the flag should contain squares of the same colour, and the colours of adjacent horizontal rows should be different. Berland's government asked you to find out whether their flag meets the new ISO standard.

输入格式

The first line of the input contains numbers $n$ and $m$ ( $1<=n,m<=100$ ), $n$ — the amount of rows, $m$ — the amount of columns on the flag of Berland. Then there follows the description of the flag: each of the following $n$ lines contain $m$ characters. Each character is a digit between 0 and 9, and stands for the colour of the corresponding square.

输出格式

Output YES, if the flag meets the new ISO standard, and NO otherwise.

输入输出样例

输入#1
```
3 3
000
111
222
```
输出#1
```
YES
```
输入#2
```
3 3
000
000
111
```
输出#2
```
NO
```
输入#3
```
3 3
000
111
002
```
输出#3
```
NO
```

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