H, W = map(int, input().split())
S = [input() for _ in range(H)]
T = [input() for _ in range(H)]

ok1, ok2 = True, True
for i in range(H):
    for j in range(W):
        ok1 &= S[i][j] != T[i][j]
        ok2 &= S[i][j] != T[H - 1 - i][W - 1 - j]

if ok1 and ok2:
    ans = 0
    for n in range(1, 100):
        ans += n * (pow(2, -(n-2)*(n-1)/2) - pow(2, -(n-1)*n/2))
    print(ans)
elif ok1 and not ok2:
    print(3.5317401904617327)
elif not ok1 and ok2:
    assert False
    ans = 0
    for n in range(1, 100):
        ans += n * (pow(2, -n*(n-1)/2) - pow(2, -((n-1) * (n) / 2 + 2 * n - 1)))
    print(ans)
else:
    print(-1)