h,w = input().split()
h,w = int(h),int(w)
lst=[]

def f(plus_index):
    
    dummy = lst[:]
    h_index = lst[plus_index][0]-lst[0][0]
    w_index = lst[plus_index][1]-lst[0][1]
    
    for i in dummy:
        n = [i[0]+h_index,i[1]+w_index]
        if n in dummy:
            dummy.remove(n)
        else:
            return False
    else:
        print("YES")
        return True

for i in range(h):
    for j,x in enumerate(list(input())):
        if x=="#":
            lst.append([i,j])

if len(lst)%2==1:
    print("NO")
else:
    for i in range(1,len(lst)//2+1):
        if f(i):
            break
    else:
        print("NO")