h, w = map(int, input().split())
s = [input() for _ in range(h)]
t = [input() for _ in range(h)]
ok1 = all(s[i][j] != t[i][j] for i in range(h) for j in range(w))
for i in range(h):
    s[i] = ''.join(reversed(s[i]))
s.reverse()
ok2 = all(s[i][j] != t[i][j] for i in range(h) for j in range(w))
if not ok1 and not ok2:
    print(-1)
    exit()
ans = 0.0
p = 1.0
i = 0
while p:
    ans += p
    if ok1:
        for _ in range(i):
            p /= 2
    i += 1
    ok1, ok2 = ok2, ok1
print(f'{ans:.16f}')