1018 python

import sys
import math

n, m = map(int, sys.stdin.readline().split())
arr = [sys.stdin.readline().strip() for _ in range(n)]

minV = math.inf

for i in range(n-7):
    for j in range(m-7):
        w, b = 0, 0
        for y in range(i, i+8):
            for x in range(j, j+8):
                if (x+y)%2 == 0:
                    if arr[y][x] != 'W':
                        w += 1
                    else:
                        b += 1
                else:
                    if arr[y][x] != 'B':
                        w += 1
                    else:
                        b += 1
        if w < b:
            if w < minV:
                minV = w
        else:
            if b < minV:
                minV = b
print(minV)