H, W = map(int, input().strip().split(' '))

odd = 0
even = 0

for i in range(H):
    row = input().strip()
    for j in range(W):
        if row[j] != 'o':
            continue

        state = H - i + j
        if state % 2 == 0:
            even += 1
        else:
            odd += 1

if odd % 2 == 0:
    if even % 2 == 0:
        print('YES')
    else:
        print('NO')
else:
    if even % 2 == 0:
        print('NO')
    else:
        if odd == 1:
            print('NO')
        else:
            print('YES')