#include<iostream>
using namespace std;
char arr[1000][1000];
long long w[100000], b[100000], r[100000];
long long ans = 1e9;
int main() {
    int n, m;
    cin >> n >> m;
    for (long long i = 1; i <= n; i++)
        for (long long j = 1; j <= m; j++) {
            cin >> arr[i][j];
            if (arr[i][j] != 'W') w[i]++;
            if (arr[i][j] != 'B') b[i]++;
            if (arr[i][j] != 'R') r[i]++;
        }
    for (int i = 2; i <= n; i++) {
        w[i] += w[i - 1];
        b[i] += b[i - 1];
        r[i] += r[i - 1];
    }
    for (int i = 1; i <= n - 2; i++)
        for (int j = i + 1; j <= n; j++)
            ans = min(ans, w[i] + b[j] - b[i] + r[n] - r[j]);
    cout << ans;
    return 0;
}