import sys
input=sys.stdin.readline
n,m=map(int, input().split())
arr=[]
res=[]
for _ in range(n):
arr.append(input().rstrip())
for i in range(n-7):
for j in range(m-7):
W=0
B=0
for a in range(i, i+8):
for b in range(j, j+8):
if (a+b)%2==0:
if arr[a][b]!='W':
W+=1
else:
B+=1
else:
if arr[a][b]!='W':
B+=1
else:
W+=1
res.append(W)
res.append(B)
print(min(res))