全WA版

#include<iostream>
#include<vector>
#define F(i,a,b) for( int i=(a); i<=(b); ++i)
using namespace std;
vector<vector<int>> flag(51, vector<int>(3));
int N, M;
int main() {
	cin >> N >> M;
	F(i, 1, N) {
		auto& fg = flag[i], & fg_l = flag[i - 1];
		F(ii, 0, M) {
			switch (cin.get())
			{
			case 'W':
				fg[0]++; break;
			case 'B':
				fg[1]++; break;
			case 'R':
				fg[2]++; break;
			}
		}
		fg[0] += fg_l[0];
		fg[1] += fg_l[1];
		fg[2] += fg_l[2];
	}
	unsigned int ans = M * N;
	const int flag_n2 = flag[N][2], mn = M * N;
	F(i, 1, N - 2) {
		unsigned int temp_ans_1 = mn - flag[i][0] + flag[i][1];
		F(ii, i + 1, N - 1) {
			unsigned int temp_ans = temp_ans_1 + flag[ii][2] - flag[ii][1] - flag_n2;
			ans = ans < temp_ans ? ans : temp_ans;
		}
	}
	cout << ans;
}

switch (cin.get()) 
↓
char c;cin>>c;
switch (c)

AC两个
然后整个输入换成

//F(ii, 0, M) {
	string s; cin >> s;
	for(char c:s)
	switch (c)
	{...}
//}

全AC
更离谱的是

F(ii, 0, M) {
	string s; cin >> s;
	for(char c:s)
	switch (c)
	{...}
}

会全WA而不是TLE