956 B | 956 chars
n,m=map(int,input().split())
L = [list(map(int,list(input()))) for i in range(n)]
# print(L)
check = True
cnt = 0
for i in range(n):
for j in range(m):
k = L[i][j]
k2 = L[n-i-1][m-j-1]
if i == n-i-1 and j == m-j-1:
if k == k2:
if k != 8:
check = False
if k == 7:
if k2 == 7:
cnt+=1
else:
check = False
elif k == 6:
if k2 == 6:
cnt+=1
elif k2 == 9:
continue
else:
check = False
elif k == 8:
if k2 == 8:
continue
else:
check = False
elif k == 9:
if k2 == 6:
continue
elif k2 == 9:
cnt+=1
else:
check = False
if check:
print(cnt//2)
else:
print(-1)