//#pragma GCC optimize("Ofast,no-stack-protector")
 //#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("Ofast")
#pragma GCC optimize("fast-math")
 /*#pragma GCC optimize("section-anchors")
 #pragma GCC optimize("profile-values,profile-reorder-functions,tracer")
 #pragma GCC optimize("vpt")
 #pragma GCC optimize("rename-registers")
 #pragma GCC optimize("move-loop-invariants")
 #pragma GCC optimize("unswitch-loops")
 #pragma GCC optimize("function-sections")
 #pragma GCC optimize("data-sections")
 #pragma GCC optimize("branch-target-load-optimize")
 #pragma GCC optimize("branch-target-load-optimize2")
 #pragma GCC optimize("btr-bb-exclusive")*/
 //#pragma GCC optimize("inline")
 //#pragma comment(linker, "/STACK:167077260")
#define _CRT_SECURE_NO_WARNINGS
#include <chrono>
#include <set>
#include <map>
#include <deque>
#include <string>
#include <cstdint>
#include <cmath>
#include <queue>
#include <cassert>
#include <random>
#include <bitset>
#include <iomanip>
#include <numeric>
#include <time.h>//////////////
#include <ctime>
#include <climits>
#include <string>
#include <cstdio>
#include <vector>
#include <cstdlib>
#include <iostream>
#include <algorithm>
#include <unordered_map>
#include <unordered_set>
 //++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
 //#define endl '\n'
#define mp make_pair
#define pbc push_back
#define pob pop_back()
#define empb emplace_back
#define queuel queue<long long>
#define sqr(a) ((ld)(a) * (a))
#define lsqr(a) (ld(a) * a)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define pin(p) cin >> p.first >> p.second;
#define uniq(a) sort(all(a));(a).resize(unique(all(a)) - a.begin());
#define rev(v) reverse(v.begin(), v.end());
#define sout(s, c) for (auto i : s) cout << i << c;
#define pout(p) cout << p.first << " " << p.second;
#define er(v, l, r) erase(v.begin() + l, v.begin() + r);
#define vin(v) for (ll i = 0; i < v.size(); ++i) cin >> v[i];
#define vout(v, c) for (int i = 0; i < v.size(); ++i) cout << v[i] << c;
#define pushi(v, a) for (int i = 0; i < a.size(); ++i) v.push_back(a[i]);
#define fastio() ios_base::sync_with_stdio(0); cout.tie(0); cin.tie(0); srand(time(NULL))
#define dab(v) for(auto i:v)cout<<i<<' ';
#define sp system("pause")
#define left left228
#define type type1337
#define right right228
 //++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
	using namespace std;
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
//++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++--++
const ld EPS = 1e-5;
const ld EPS2 = 1e-12;
const ld PI = acos(-1);
int mod = (int)998244353;
const int MOD7 = 1000000007;
const int MOD9 = 1000000009;
const int a228 = 18;
const int inf = 1e9;
mt19937 rnd(time(0));
const ll kekmod = 1791791791;
const ll bestmod = 1148822869;
int n;
const int MAXN = 4e3 + 1;
int a[MAXN];
int place[MAXN][2];
char c[MAXN];
pair<int, int> pref[MAXN];
int cnts[MAXN][MAXN][2];
int cnt[MAXN][2];
signed main()
{
	fastio();
	cin >> n;
	n *= 2;
	for (int i = 0; i < n; ++i)
	{
		cin >> c[i] >> a[i];
		if (c[i] == 'W') c[i] = 0;
		else c[i] = 1;
	}
	for (int i = 0; i < n; ++i)
	{
		pair<int, int> las = (i == 0 ? mp(0, 0) : pref[i - 1]);
		if (c[i] == 1) las.second++;
		else las.first++;
		cnt[a[i]][c[i]]++;
		pref[i] = las;
		if (c[i] == 1) place[a[i]][1] = i;
		else place[a[i]][0] = i;
		for (int j = 0; j < MAXN; ++j) for (int x = 0; x < 2; ++x) cnts[i][j][x] = cnt[j][x];
	}
	for (int i = 0; i < n; ++i)
	{
		for (int j = 1; j < MAXN; ++j)
		{
			for (int x = 0; x < 2; ++x)
			{
				cnts[i][j][x] += cnts[i][j - 1][x];
			}
		}
	}
	vector<vector<int>> dp(n + 2, vector<int>(n + 2,inf));
	dp[0][0] = 0;
	for (int i = 0; i <= n/2; ++i)
	{
		for (int j = 0; j <= n/2; ++j)
		{
			int hv = i + j;
			int take = place[i + 1][0] - cnts[place[i+1][0]][i][0] - cnts[place[i+1][0]][j][1];
			dp[i + 1][j] = min(dp[i + 1][j], dp[i][j] + take);
			take = place[j + 1][1] - cnts[place[j+1][1]][i][0] - cnts[place[j+1][1]][j][1];
			dp[i][j + 1] = min(dp[i][j + 1], dp[i][j] + take);
		}
	}
	cout << dp[n / 2][n / 2];
	sp;
}

Measured memory_bytes: 190856