def res():
    H,W=map(int,input().split())
    S=[[None] for _ in range(H)]

    for i in range(H):
        S[i]=list(input())

    b=[]
    for i in range(H):
        for j in range(W):
            if S[i][j]=='#':
                b.append([i,j])

    if (len(b)>1)and(len(b)%2==0):
        o=b[0]
        for i in b:
            l1=i[0]-o[0]
            l2=i[1]-o[1]
            t=[i for i in b]
            while t:
                j=t.pop(0)
                if [j[0]+l1,j[1]+l2]in t:
                    t.remove([j[0]+l1,j[1]+l2])
                else:
                        break
            if len(t)==0:
                return 'YES'
        return 'NO'
    return 'NO'

print(res())