#include <iostream>

using namespace std;

string chess1[8];

string chess2[8];

int N, M;

string board[50];

int ans = 999999999;

int main()

{

int i, j, x, y;

for (i = 0; i < 8; i++) {

if (i % 2 == 0) {

chess1[i] = "WBWBWBWB";

chess2[i] = "BWBWBWBW";

}

else {

chess1[i] = "BWBWBWBW";

chess2[i] = "WBWBWBWB";

}

}

cin >> N >> M;

for (i = 0; i < N; i++) {

cin >> board[i];

}

for (i = 0; i < N - 7; i++) {

for (j = 0; j < M - 7; j++) {

int count1 = 0;

int count2 = 0;

for (x = 0; x < 8; x++) {

for (y = 0; y < 8; y++) {

if (board[i + x][j + y] != chess1[x][y]) count1++;

if (board[i + x][j + y] != chess2[x][y]) count2++;

}

}

if (count1 < ans) ans = count1;

if (count2 < ans) ans = count2;

}

}

cout << ans;

return 0;

}