N, M = map(int, input().split())
S = [input() for i in range(N)]

v = [sum(1 << j for j in range(M) if S[i][j] == 'o') for i in range(N)]

def popcount(bit):
	return bin(bit).count('1')

ans = 0
dp = [0] * (1 << N)
dp[0] = 1
for bit in range(1 << N):
	vand = (1 << M) - 1
	for i in range(N):
		if bit >> i & 1:
			vand &= v[i]
			
	if popcount(vand) == 1:
		ans += dp[bit] * popcount(bit)
		continue
	
	for i in range(N):
		if bit >> i & 1:
			continue
		dp[bit | (1 << i)] += dp[bit] / (N - popcount(bit))

print(f"{ans:.15f}")