BAEKJOON_1018

import java.io.*;
import java.util.*;


public class Main {
    static boolean[][] arr;
    public static int min =64;
    public static void main(String[] args) throws IOException {

        BufferedReader br =new BufferedReader(new InputStreamReader(System.in));
        BufferedWriter bw =new BufferedWriter(new OutputStreamWriter(System.out));

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

        arr=new boolean[N][M];
        //StringBuilder sb;
        for(int i=0; i<N; i++){
            String sb =br.readLine();
            for(int j=0; j<M; j++){
                if(sb.charAt(j)=='W'){
                    arr[i][j]=true;
                }else{
                    arr[i][j] =false;
                }
            }
        }

        int x = N-7;        //N =10
        int y=  M-7;

        for(int i=0; i<x; i++){
            for(int j=0; j<y; j++){
                find(i,j);
            }
        }
        System.out.println(min);
    }
    public static void find(int x,int y){
        int x2 = x+8;
        int y2 = y+8;

        boolean test =arr[x][y];
        int count=0;
        for(int i=x; i<x2; i++){
            for(int j=y; j<y2; j++){
                if(arr[i][j] != test){
                    count++;
                }

                test = (!test);
            }
            test = !test;
        }
        count= Math.min(count, 64-count);

        min = Math.min(min,count);
    }
}