import sys
readline=sys.stdin.readline



H,W=map(int,readline().split())
g=0
c=0
for h in range(H):
    S=readline().rstrip()
    for w in range(W):
        if S[w]=="o":
            c+=1
            if (h+w)%2==(H-1)%2:
                g+=1
            else:
                g+=2
if g%3:
    ans="NO"
else:
    ans="YES"
print(ans)