CF462A Appleman and Easy Task

Toastman came up with a very easy task. He gives it to Appleman, but Appleman doesn't know how to solve it. Can you help him? Given a $ n×n $ checkerboard. Each cell of the board has either character 'x', or character 'o'. Is it true that each cell of the board has even number of adjacent cells with 'o'? Two cells of the board are adjacent if they share a side.

The first line contains an integer $ n $ ( $ 1

Print "YES" or "NO" (without the quotes) depending on the answer to the problem.