h,w = map(int,input().split())
b = [input() for _ in range(h)]
r = [0,0]
cnt = 0
for i in range(h):
    for j in range(w):
        if b[i][j] == "o":
            cnt += 1
            r[(i+j)%2] += 1
print("YES" if (cnt+r[0])%3==0 else "NO")