import java.util.ArrayList;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int x = sc.nextInt();
		int y = sc.nextInt();
		ArrayList<String[]> al = new ArrayList<String[]>();
		int arr1[] = new int[x];
		int arr2[] = new int[x];
		int arr3[] = new int[x];
		for(int i = 0;i < x;i++) {
			String arr[] = sc.next().split("");
			al.add(arr);
			arr1[i] = 0;
			arr2[i] = 0;
			arr3[i] = 0;
		}
		int count = 0;
		int count1 = 9999;
		for(String arr[] : al) {
			for(int i = 0;i <= arr.length - 1;i++) {
				if(arr[i].equals("W")) {

				}else {
					arr1[count]++;
				}
				if(arr[i].equals("B")) {
					
				}else {
					arr2[count]++;
				}
				if(arr[i].equals("R")) {
					
				}else {
					arr3[count]++;
				}
			}
			count++;
		}
		count = 0;
		for(int i = 2;i <= x - 1;i++) {
			for(int j = 0;j < i;j++) {
				if(j == 0) {
					count += arr1[j];
				}else if(j == i - 1) {
					count += arr2[j];
				}else {
					if(arr1[j] < arr2[j]) {
						count += arr1[j];
					}else {
						count += arr2[j];
					}
				}
			}
			for(int j = i;j <= x - 1;j++) {
				count += arr3[j];
			}
			if(count < count1) {
				count1 = count;
			}
			count = 0;
		}
		System.out.println(count1);
	}
}