package Q1018;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class Q1018 {

    public static void main(String[] args) throws Exception {
        BufferedReapackage Q1018;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

        public class Q1018 {

            public static void main(String[] args) throws Exception {
                BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

                StringTokenizer st = new StringTokenizer(br.readLine());
                int N = Integer.parseInt(st.nextToken());
                int M = Integer.parseInt(st.nextToken());

                char[][] map = new char[N][M];
                for (int i = 0; i < N; i++) {
                    String line = br.readLine();
                    map[i] = line.toCharArray();
                }

                int minCount = 64;

                for (int y = 0; y <= N - 8; y++) {
                    for (int x = 0; x <= M - 8; x++) {
                        int countW = 0;
                        for (int i = y; i < y + 8; i++) {
                            for (int j = x; j < x + 8; j++) {

                                char expectedChar;
                                if ((i + j) % 2 == 0) {
                                    expectedChar = 'W';
                                } else {
                                    expectedChar = 'B';
                                }

                                if (map[i][j] != expectedChar) {
                                    countW++;
                                }
                            }
                        }

                        int countB = 0;
                        for (int i = y; i < y + 8; i++) {
                            for (int j = x; j < x + 8; j++) {
                                char expectedChar;
                                if ((i + j) % 2 == 0) {
                                    expectedChar = 'B';
                                } else {
                                    expectedChar = 'W';
                                }
                                if (map[i][j] != expectedChar) {
                                    countB++;
                                }
                            }
                        }

                        int min = Math.min(countW, countB);
                        if (min < minCount) {
                            minCount = min;
                        }
                    }
                }

                System.out.println(minCount);
            }
        }
        der br = new BufferedReader(new InputStreamReader(System.in));

        StringTokenizer st = new StringTokenizer(br.readLine());
        int N = Integer.parseInt(st.nextToken());
        int M = Integer.parseInt(st.nextToken());

        char[][] map = new char[N][M];
        for (int i = 0; i < N; i++) {
            String line = br.readLine();
            map[i] = line.toCharArray();
        }

        int minCount = 64;

        for (int y = 0; y <= N - 8; y++) {
            for (int x = 0; x <= M - 8; x++) {
                int countW = 0;
                for (int i = y; i < y + 8; i++) {
                    for (int j = x; j < x + 8; j++) {

                        char expectedChar;
                        if ((i + j) % 2 == 0) {
                            expectedChar = 'W';
                        } else {
                            expectedChar = 'B';
                        }

                        if (map[i][j] != expectedChar) {
                            countW++;
                        }
                    }
                }

                int countB = 0;
                for (int i = y; i < y + 8; i++) {
                    for (int j = x; j < x + 8; j++) {
                        char expectedChar;
                        if ((i + j) % 2 == 0) {
                            expectedChar = 'B';
                        } else {
                            expectedChar = 'W';
                        }
                        if (map[i][j] != expectedChar) {
                            countB++;
                        }
                    }
                }

                int min = Math.min(countW, countB);
                if (min < minCount) {
                    minCount = min;
                }
            }
        }

        System.out.println(minCount);
    }
}