import sys
input = sys.stdin.readline
H, W = map(int, input().split())
S = [input().strip() for _ in range(H)]
T = [input().strip() for _ in range(H)]
Srev = [["" for _ in range(W)] for _ in range(H)]
for i in range(H):
    for j in range(W):
        Srev[H-1-i][W-1-j] = S[i][j]
flag = True
for i in range(H):
    for j in range(W):
        if S[i][j]==T[i][j]:
            flag = False
            break
    if flag==False:
        break
flagrev = True
for i in range(H):
    for j in range(W):
        if Srev[i][j]==T[i][j]:
            flagrev = False
            break
    if flagrev==False:
        break
if flag and flagrev:
    ans = 0
    anspre = -1
    i = 1
    fail = 1
    while ans-anspre>10**(-7):
        anspre = ans
        f = 1/(2**(i-1))
        ans += i*(1-f)*fail
        fail *= f
        i+=1
    print(ans)
elif flag:
    ans = 0
    anspre = -1
    i = 1
    fail = 1
    while ans-anspre>10**(-7):
        if i%2==1:
            f = 1/(2**(i-1))
        else:
            f = 1
        if f<1:
            anspre = ans
        ans += i*(1-f)*fail
        fail *= f
        i+=1
    print(ans)
elif flagrev:
    ans = 0
    anspre = -1
    i = 1
    fail = 1
    while ans-anspre>10**(-7):
        if i%2==0:
            f = 1/(2**(i-1))
        else:
            f = 1
        if f<1:
            anspre = ans
        ans += i*(1-f)*fail
        fail *= f
        i+=1
    print(ans)
else:
    print(-1)