#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define ull unsigned long long
#define db double
int w[55],b[55],r[55];
char col[55][55];
int n,m,res=5000;
void dfs(int index,int sum,int flag){
    if(index==n&&flag==2){
        res=min(res,sum);
        return;
    }
    if(index>=n){
        return;
    }
    int i=index;
    if(flag==0){
        dfs(i+1,sum+w[i],0);
        dfs(i+1,sum+b[i],1);
    }
    else if(flag==1){
        dfs(i+1,sum+b[i],1);
        dfs(i+1,sum+r[i],2);
    }
    else{
        dfs(i+1,sum+r[i],2);
    }
}
void solve(){
    memset(w,0,sizeof(w));
    memset(b,0,sizeof(b));
    memset(r,0,sizeof(r));
    cin>>n>>m;
    for(int i=0;i<n;i++){
        for(int j=0;j<m;j++){
            cin>>col[i][j];
            if(col[i][j]!='W'){
                w[i]++;
            }
            if(col[i][j]!='B'){
                b[i]++;
            }
            if(col[i][j]!='R'){
                r[i]++;
            }
        }
    }
    dfs(0,0,0);
    cout<<res;
}
int main(){
    cin.tie(0);
    cout.tie(0);
    ios::sync_with_stdio(0);
    int T=1;
    //cin >> T;
    while(T--){
        solve();
    }
    return 0;
}