#include<bits/stdc++.h>
#define int long long
#define d lld
using namespace std;
const int N=60;
int n,m,sum,ans;
char a[N][N];
signed main(){
	cin>>n>>m;
	for(int i=1;i<=n;++i)
		for(int j=1;j<=m;++j)
			cin>>a[i][j];
	for(int w=1;w<=n-2;++w){
		sum=0;
		for(int i=1;i<=w;++i){
			for(int j=1;j<=m;++j){
				if(a[i][j]!='W') sum++;
			}
		}
		for(int b=1;b<=n-w-1;++b){
			for(int i=1;i<=b;++i){
				for(int j=1;j<=m;++j){
					if(a[i+w][j]!='B') sum++;
				}
			}
			int r=n-w-b;
			for(int i=1;i<=r;++i){
				for(int j=1;j<=m;++j){
					if(a[i+w+b][j]!='R') sum++;
				}
			}
		}
		if(ans) ans=min(ans,sum);
		else ans=sum;
	}
	cout<<ans<<endl;
	return 0;
}