H, W = map(int, input().split())
grid = [input().strip() for _ in range(H)]

black_cells = [(i, j) for i in range(H) for j in range(W) if grid[i][j] == '#']
n = len(black_cells)

if n < 2:
    print("NO")
    exit()

black_set = set(black_cells)
dxdy = set()

for i in range(n):
    ax, ay = black_cells[i]
    for j in range(n):
        if i != j:
            bx, by = black_cells[j]
            dx = bx - ax
            dy = by - ay
            dxdy.add((dx, dy))

found = False

for dx, dy in dxdy:
    R = set()
    for (x, y) in black_cells:
        nx = x + dx
        ny = y + dy
        if (nx, ny) in black_set:
            R.add((x, y))
    if not R:
        continue
    B = {(x + dx, y + dy) for (x, y) in R}
    if R & B:
        continue
    total = R.union(B)
    if len(total) == n and total == black_set:
        print("YES")
        found = True
        break

if not found:
    print("NO")