#include<iostream>
using namespace std;
char ch[52][52];
enum Color{W,B,R}color;

int count(char c, char* a)
{
	int ret=0;
	while((*a)!=0){
		if((*a)==c)ret++;
		a++;
	}
	return ret;
}

int main()
{
	int n, m, ans=0;cin>>n>>m;
	for(int i=0; i<n; i++)for(int j=0; j<m; j++)cin>>ch[i][j];
	for(int i=0; i<n; i++){
		int w=count('W', ch[i]);
		int b=count('B', ch[i]);
		int r=count('R', ch[i]);
		switch(color)
		{
			case W:
				if(w<=b)ans+=m-w;else color=B, ans+=m-w;
				break;
			case B:
				if(b<=r&&i!=n-1)ans+=m-b;else color=R, ans+=m-b;
				break;
			case R:
				ans+=m-r;
				break;
		}
	}
	cout<<ans;
	return 0;
}