def main():
    h, w = list(map(int, input().split()))
    S = [input() for _ in range(h)]
    for x in range(w):
        for y in range(h):
            if S[y][x] == "x":
                break
        else:
            mx = x
    A = [0] * h
    for y in range(h):
        for x in range(w):
            nx = x
            if x == mx:
                continue
            elif mx < x:
                nx -= 1
            if S[y][x] == "x":
                A[y] |= 1<<nx
    dp = [0] * (1<<h)
    for y in range(h):
        dp[1<<y] = A[y]
    X = [0] * (h+1)
    X[0] = 1
    for y in range(1, h+1):
        X[y] = X[y-1]*(h+1-y)//y
    o = 1<<60
    ans = o
    mask = (1<<w-1)-1
    for bit in range(1, 1<<h):
        mbit = bit & (bit-1)
        dp[bit] = dp[mbit] | dp[mbit^bit]
        if dp[bit] < mask:
            c = bin(bit).count("1")
            ans += o//X[c]
    return ans/o

print(main())