H,W = map(int,input().split())
S = []
for i in range(H):
  S.append(input())
def translation(px,py):
  C = [[0 for x in range(W)] for y in range(H)]
  for y in range(H):
    for x in range(W):
      if C[y][x] == 0:
        C[y][x] = 1
        if x + px < W and y + py < H:
          if S[y][x] != S[y + py][x + px]:
            return False
          C[y + py][x + px] = 1
        else:
          if S[y][x] != '.':
            return False
  return True
def check():
  for y in range(H):
    for x in range(W):
      if not(x == 0 and y == 0 ):
        if translation(x,y):
          return 'YES'
  return 'NO'
print(check())