第一行必W，最后一行必R，对中间的各行符号进行枚举，固定了顺序且去了重复

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int n, m;
char color[5] = {'W', 'B', 'R'};
char a[55][55];
vector<int> result;
bool visit[55];
int last;
int cnt;
int temp_breadth;
int temp_depth;
vector<char> check;
void dfs(int depth, int last);
int main() {
	cin >> n >> m;
	for (int i = 1; i <= n; i++) {
		for (int j = 1; j <= m; j++) {
			cin >> a[i][j];
		}
	}
	dfs(0, 0);
	int row1 = 0, rown = 0;
	for (int i = 1; i <= m; i++) {
		if (a[1][i] != 'W') {
			row1++;
		}
		if (a[n][i] != 'R') {
			rown++;
		}
	}
	sort(result.begin(), result.end());
	cout << result[0] + row1 + rown;
	return 0;
}
bool is_flag(vector<char> &check) {
	for (int i = 0; i <= check.size() - 1; i++) {
		if (check[i] == 'B') {
			return true;
		}
	}
	return false;
}
void dfs(int depth, int last) {
	if (depth >= n - 2) {
		if (is_flag(check)) {
			result.push_back(cnt);
		}
		return ;
	}
	char last_c = '0';
	for (int i = last; i <= 2; i++) {
		if (color[i] != last_c ) {
			for (int j = 1; j <= m; j++) {
				if (a[depth + 2][j] != color[i]) {
					cnt++;
				}
			}
			check.push_back(color[i]);
			last_c = color[i];
			temp_depth = cnt;
			dfs(depth + 1, i);
			cnt = temp_depth;
			check.pop_back();
		}
	}
}