N,M=map(int,input().split())
l=[]
for i in range(N):
    S=[list(input()) for i in range(M)]
    l.append(S)

def to_base(n,base,leng):
    digit='0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    if n==0:
        return '0'*leng
    res=[]
    while n:
        res.append(digit[n%base])
        n//=base
    res+=['0']*(max(0,leng-len(res)))
    return ''.join(res[::-1])

#左90°回転
def rot(S):
    return [list(r) for r in zip(*S)][::-1]

ans=10**18
for n in range(4**N):
    cur=[[False]*M for i in range(M)]
    b=to_base(n,4,N)
    for j in range(N):
        G=l[j]
        for _ in range(int(b[j])):
            G=rot(G)
        for r in range(M):
            for c in range(M):
                if G[r][c]=='#':
                    cur[r][c]=True
    ans=min(ans,sum(sum(v) for v in cur))

print(ans)