P3392 涂国旗

// Problem: P3392 涂国旗
// Contest: Luogu
// URL: https://www.luogu.com.cn/problem/P3392
// Memory Limit: 125 MB
// Time Limit: 1000 ms
// User: Pannnn
 
#include <bits/stdc++.h>
 
using namespace std;
 
int cntDiff(vector<string> &info, int row1, int row2) {
    int cnt = 0;
    for (int i = 0; i <= row1; ++i) {
        for (int j = 0; j < info[i].size(); ++j) {
            if (info[i][j] != 'W') {
                ++cnt;
            }
        }
    }
    
    for (int i = row1 + 1; i <= row2; ++i) {
        for (int j = 0; j < info[i].size(); ++j) {
            if (info[i][j] != 'B') {
                ++cnt;
            }
        }
    }
    
    for (int i = row2 + 1; i < info.size(); ++i) {
        for (int j = 0; j < info[i].size(); ++j) {
            if (info[i][j] != 'R') {
                ++cnt;
            }
        }
    }
    return cnt;
}
 
int getMinCnt(vector<string> &info) {
    int res = INT_MAX;
    // 遍历所有格子划分情况，计算要修改多少次，注意，每一行至少为1
    for (int i = 0; i < info.size(); ++i) {
        for (int j = i + 1; j < info.size() - 1; ++j) {
            res = min(res, cntDiff(info, i, j));
        }
    }
    return res;
}
 
int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    
    int n, m;
    cin >> n >> m;
    
    vector<string> info(n);
    for (int i = 0; i < n; ++i) {
        cin >> info[i];
    }
    int res = getMinCnt(info);
    cout << res << endl;
    
    return 0;
}