N, M = map(int, input().split())
S = [int(input().replace("o", "0").replace("x", "1"), 2) for i in range(N)]
ok = 0
for s in S:
    ok |= s
dp = [False for i in range(2**N)]
pp = [0 for i in range(2**N)]
pp[0] = 1
ans = 0
for i in range(2**N):
    t = 0
    c = i.bit_count()
    for j in range(N):
        if (i >> j) & 1:
            t |= S[j]
            k = i ^ (1 << j)
            if not dp[k]:
                pp[i] += pp[k] / (N - c + 1)
    if t == ok:
        dp[i] = True
        ans += c * pp[i]
print(ans)