import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
import numpy as np

H, W = map(int, readline().split())

# 1次元の場合を実験して観察すると解ける
S = np.frombuffer(read(), 'S1').reshape(H, -1)[:, :W]
S = 1 * (S == b'o')
x = S[::2, ::2].sum() + S[1::2, 1::2].sum()
y = S.sum() - x

print('NO' if (x - y) % 3 else 'YES')