#include <iostream>
using namespace std;
//枚举变色的分界线O(n^n) 
int n,m,cnt,ans = 3000;
char Map[60][60];
int main(){
	cin>>n>>m;
	for(int i = 1; i <= n; i++)
		for(int j = 1; j <= m; j++)
			cin>>Map[i][j];
	for(int x = 2; x <= n-1; x++){//白--蓝
		cnt = 0; 
		for(int y = x+1; y <= n; y++){//蓝--红 
			for(int i = 1; i <= n; i++){
				for(int j = 1; j <= m; j++){
					if(i >= 1 && i < x && Map[i][j] != 'W') cnt++;//白蓝分界线之前变白
					if(i >= x && i < y && Map[i][j] != 'B') cnt++;//蓝红分界线前变蓝
					if(i >= y && Map[i][j] != 'R') cnt++;//蓝红分界线之后变红 
				}
			}
			ans = min(ans,cnt);
		}
	}
	cout<<ans;
	return 0;
}