Flag

Time Limit: 2 seconds

Memory Limit: 64 megabytes

Description

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.

Sample Input

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

Sample Output

None

Hint

None

Source

None

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