N, M = map(int, input().split())
S = [[input() for j in range(M)] for i in range(N)]
v = [[(j, k) for j in range(M) for k in range(M) if S[i][j][k] == '#'] for i in range(N)]

vt = [[set(v[i]), set(), set(), set()] for i in range(N)]
for i in range(N):
	for t in range(3):
		for p in vt[i][t]:
			vt[i][t + 1].add((p[1], M - 1 - p[0]))

ans = 10 ** 9
for bit in range(1 << ((N - 1) * 2)):
	s = set()
	for i in range(N):
		t = bit >> (i * 2) & 3
		s = s | vt[i][t]
	ans = min(ans, len(s))
	
print(ans)