#include <stdio.h>
#define min(a,b) (a)<(b)?(a):(b)
int n, m, i, j, a, b, ans = 1e9;
char str[55][55];
 
int main() {
	scanf("%d %d", &n, &m);
	for (i = 0; i < n; i++) scanf("%s", str[i]);
 
	for (i = 0; i < n - 7; i++) for (j = 0; j < m - 7; j++) {
		int cnt = 0;
		for (a = i; a < i + 8; a++) for (b = j; b < j + 8; b++) {
			cnt += str[a][b] == ((a&1)^(b&1) ? 'W':'B');
		}
		ans = min(ans, min(cnt, 64 - cnt));
	}
 
	printf("%d", ans);
 
	return 0;
}

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

OCA4CldCV0JXQldCCkJXQldCV0JXCldCV0JXQldCCkJXQkJCV0JXCldCV0JXQldCCkJXQldCV0JXCldCV0JXQldCCkJXQldCV0JX

8 8
WBWBWBWB
BWBWBWBW
WBWBWBWB
BWBBBWBW
WBWBWBWB
BWBWBWBW
WBWBWBWB
BWBWBWBW