rt：

#include <bits/stdc++.h>
using namespace std;
int n,m,ans=0x3f;
char a[600][600];
void find (int x,int y) {
	int s=0;
	for (int i=1;i<=x;i++) 
	    for (int j=1;j<=m;j++) s+=(a[i][j]!='W');
	for (int i=x+1;i<=y;i++) 
	    for (int j=1;j<=m;j++) s+=(a[i][j]!='B');
	for (int i=y+1;i<=n;i++) 
	    for (int j=1;j<=m;j++) s+=(a[i][j]!='R');
	ans=min (ans,s);
}
int main() {
    cin>>n>>m;
    for (int i=1;i<=n;i++) 
        for (int j=1;j<=m;j++) cin>>a[i][j];
    for (int i=1;i<=n-2;i++) 
        for (int j=i+1;j<=n-1;j++) find (i,j);
    cout<<ans;
	return 0;
}