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)]

pcnt = [0] * (1 << N); vand = [(1 << M) - 1] * (1 << N)
for i in range(N):
	for bit in range(1 << i):
		pcnt[bit | (1 << i)] = pcnt[bit] + 1
		vand[bit | (1 << i)] = vand[bit] & v[i]

ans = 0
dp = [0] * (1 << N)
dp[0] = 1
for bit in range(1 << N):
	if bin(vand[bit]).count('1') == 1:
		ans += pcnt[bit] * dp[bit]
		continue
	
	dp[bit] /= (N - pcnt[bit])
	for i in range(N):
		if bit >> i & 1:
			continue
		dp[bit | (1 << i)] += dp[bit]

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