#include <bits/stdc++.h>
#define inf 1e9
using namespace std;
int w[60], b[60], r[60], n, m, minn = inf;
char c;
int main() {
    cin >> n >> m;
    for(int i = 1; i <= n; i++)
        for(int j = 1; j <= m; j++) {
            cin >> c;
            if(c != 'W') w[i]++;
            if(c != 'B') b[i]++;
            if(c != 'R') r[i]++;
        }
    
    /*for(int i = 1; i <= n; i++) cout << w[i] << " "; cout << endl;
    for(int i = 1; i <= n; i++) cout << b[i] << " "; cout << endl;
    for(int i = 1; i <= n; i++) cout << r[i] << " "; cout << endl;*/

    for(int i = 1; i <= n; i++)
        for(int j = i + 1; j <= n; j++) {
            int now = 0;
            for(int x = 1; x <= i; x++) now += w[x];
            for(int x = i + 1; x <= j; x++) now += b[x];
            for(int x = j + 1; x <= n; x++) now += r[x];
            if(now < minn) minn = now;
        }
    cout << minn;
    return 0;
}

WA on #2,#6，求助！！