#include <stdio.h>
int main(){
    int N,M;
    char ch;
    scanf("%d %d",&N,&M);
    int White[55]={0},Blue[55]={0},Red[55]={0};
    for(int i=1;i<=N;i++){
        getchar();
        for(int j=1;j<=M;j++){
            scanf("%c",&ch);
            switch(ch){
                case 'W':White[i]++;break;
                case 'B':Blue[i]++;break;
                case 'R':Red[i]++;break;
                default:break;
            }
        }
    }
    //for(int i=1;i<=N;i++){
        //printf("%d %d %d\n",White[i],Blue[i],Red[i]);
    //}
    //return 0;
    int sum,min=10000000;
    int sum_w,sum_b,sum_r;
    for(int i=1;i<=N-2;i++){
        for(int j=i+1;j<=N-1;j++){
            sum_w=0;sum_b=0;sum_r=0;
            for(int k=1;k<=i;k++){
                sum_w+=White[k];
            }
            for(int k=i+1;k<=j;k++){
                sum_w+=Blue[k];
            }
            for(int k=j+1;k<=N;k++){
                sum_w+=Red[k];
            }
            sum=N*M-(sum_w+sum_b+sum_r);
            if(sum<min){
                min=sum;
            }
        }
    }
    printf("%d",min);
}