#include<bits/stdc++.h>
using namespace std;
char aaa[55][55];
int cost_w[55];
int cost_b[55];
int cost_r[55];
int  minn = INT_MAX;
int main() {
	int m, n;
	cin >> m >> n;
	for (int i = 1; i <= m; i++) {
		for (int j = 1; j <= n; j++) {
			char k; cin >> k;
			if (k == 'W') {
				cost_b[i]++;
				cost_r[i]++;
			}
			else if (k == 'R') {
				cost_b[i]++;
				cost_w[i]++;
			}
			else if (k == 'B') {
				cost_w[i]++;
				cost_r[i]++;
			}
			aaa[i][j] = k;
		}
	}
	int sum = 0;
		for (int i = 2; i < m; i++) {
			for (int j = i + 1; j <= m; j++) {
				sum = 0;
				for (int x = 1; x < i; x++)sum +=cost_w[x];
				for (int x = i; x < j; x++)sum += cost_b[x];
				for (int x = j; x <= n; x++)sum += cost_r[x];
				if (sum < minn)minn = sum;
			}
		}
		cout << minn;
	return 0;
}