Problem3895--Flag

3895: Flag

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Time Limit : 2.000 sec  Memory Limit : 256 MiB

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Description

time limit per test
2 seconds
memory limit per test
64 megabytes
input
standard input
output
standard output

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.

Input

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.

Output

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

Examples
Input
3 3
000
111
222
Output
YES
Input
3 3
000
000
111
Output
NO
Input
3 3
000
111
002
Output
NO

Source/Category

 
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