import sys
input = sys.stdin.readline

H,W=map(int,input().split())
MAP=[input().strip() for i in range(H)]
MAP.reverse()

ANS=0
for i in range(H):
    for j in range(W):
        if MAP[i][j]=="o":
            if (i+j)%2==0:
                ANS+=1
            else:
                ANS+=2

if ANS%3!=0:
    print("NO")
else:
    print("YES")