#include <bits/stdc++.h>
using namespace std;
const int N = 70;
char x[N][N];
int n,m;
int w[N],r[N],b[N];
int res =2501;
int check(char c,int i)
{
	int a = 0;
	for (int j=1;j<=m;j++)
	{
		if (x[i][j] != c)
			a++;
	}
	return a;
}

int main()
{
	scanf("%d %d",&n,&m);
	getchar();
	for (int i=1;i<=n;i++){
		for(int j=1;j<=m;j++){
			scanf("%c",&x[i][j]);
		}
		getchar();
	}
	w[0] = 0;b[0]=0;r[0]=0;
	for (int i=1;i<=n;i++)
	{
		w[i] = check('W',i) +w[i-1];
		r[i] = check('R',i) +r[i-1];
		b[i] = check('B',i) +b[i-1];
	}
	for (int i = 1;i<=n-2;i++)
		for(int j=i+1;j<=n-1;j++){
			res = min(res,w[i]+b[j]-b[i]+r[n]-r[j]);
		}
	printf("%d",res);
	return 0;
}