#include<bits/stdc++.h>
using namespace std;
char a[51][51];
int main()
{
    int n,m,minn=1e9+7;
    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+1;i++)
    {
    	for(int j=1;j<=n/2+1;j++)
    	{
    		int cnt=0;
    		int k=n-i-j;
    		if(k<=0)continue;
    		for(int x=1;x<=i;x++)for(int y=1;y<=m;y++)if(a[x][y]!='W')cnt++;
    		for(int x=i+1;x<=i+j;x++)for(int y=1;y<=m;y++)if(a[x][y]!='B')cnt++;
    		for(int x=i+j+1;x<=n;x++)for(int y=1;y<=m;y++)if(a[x][y]!='R')cnt++;
			minn=min(minn,cnt);
		}
	}
	cout<<minn;
    return 0;
}