// May this submission get accepted
#include <bits/stdc++.h>

// エイリアス
using  ll = long signed long;
using ull = long unsigned long;
using  ld = long double;
using namespace std;

// エイリアス (補完・コンパイルが重くなる)
// #include <boost/multiprecision/cpp_int.hpp>
// using mll = boost::multiprecision::cpp_int;

// 汎用マクロ
#define ALL_OF(x) (x).begin(), (x).end()
#define REP(i,n) for (long long i=0, i##_len=(n); i<i##_len; i++)
#define RANGE(i,is,ie) for (long long i=(is), i##_end=(ie); i<=i##_end; i++)
#define DSRNG(i,is,ie) for (long long i=(is), i##_end=(ie); i>=i##_end; i--)
#define STEP(i, is, ie, step) for (long long i=(is), i##_end=(ie), i##_step = (step); i<=i##_end; i+=i##_step)
#define UNIQUE(v) do { sort((v).begin(), (v).end()); (v).erase(unique((v).begin(), (v).end()), (v).end()); } while (false)
#define FOREACH(i,q) for (auto &i : q)
template<class T> bool chmax(T &a, const T b) { if (a < b) {a = b; return true;} return false; }
template<class T> bool chmin(T &a, const T b) { if (a > b) {a = b; return true;} return false; }
constexpr int INF = numeric_limits<int>::max();
constexpr long long LINF = numeric_limits<long long>::max();
constexpr long double EPS = 1e-10L;
#define Yes(q) ((q) ? "Yes" : "No")
#define YES(q) ((q) ? "YES" : "NO")
#define Possible(q) ((q) ? "Possible" : "Impossible")
#define POSSIBLE(q) ((q) ? "POSSIBLE" : "IMPOSSIBLE")
#define IIF(q,t,f) ((q) ? (t) : (f))
#define DUMP(q) DUMP_FUNC(q, #q, __FILE__, __LINE__)
template <typename T> void DUMP_PROC(T x) { if (is_integral<T>() || is_floating_point<T>()) cerr << "\e[32m" << x << "\e[m"; else cerr << x; }
template<> void DUMP_PROC<char>(char x) { cerr << "\e[36m\'" << x << "\'\e[m"; }
template<> void DUMP_PROC<string>(string x) { cerr << "\e[33m\"" << x << "\"\e[m"; }
template <typename T, typename U> void DUMP_PROC(pair<T, U> x) { cerr << "{"; DUMP_PROC(x.first); cerr << ", "; DUMP_PROC(x.second); cerr << "}"; }
template <typename ...T, typename U, U... Seq> void DUMP_PROC(tuple<T...> &x, integer_sequence<U, Seq...>) { (void)(int[]){(cerr << ((const char*[]){"", ", "})[!!Seq] << (DUMP_PROC(get<Seq>(x)), ""), 0)...}; }
template <typename ...T> void DUMP_PROC(tuple<T...> x) {cerr << "{"; DUMP_PROC(x, index_sequence_for<T...>()); cerr << "}";}
template <typename T> void DUMP_PROC(vector<T> x) { cerr << "["; for (auto &xi : x) { DUMP_PROC(xi); cerr << (&xi != &*x.rbegin()?", ":""); } cerr << "]"; }
template <typename T> void DUMP_FUNC(T x, const char* name, const char* fn, int ln) { cerr << "\e[32m[DEBUG]\e[m " << name << ": "; DUMP_PROC(x); cerr << " @ " << fn << "(" << ln << ")" << endl; }
template <class T> T gcd(T a, T b) { if (a < b) swap(a, b); while (b) swap(a %= b, b); return a; }
template <class T> T lcm(const T a, const T b) { return a / gcd(a, b) * b; }

// gcc拡張マクロ
#define popcount __builtin_popcount
#define popcountll __builtin_popcountll

// 標準入出力
struct qin { // query input
    size_t sz;
    qin(size_t _sz = 1) : sz(_sz) {}
    template <typename T> operator T () const { T a; cin >> a; return a; }
    template <typename T> operator vector<T> () const { vector<T> a(sz); for (size_t i = 0; i < sz; i++) cin >> a[i]; return a; }
    template <typename T, typename U> operator pair<T, U> () const { T f; U s; cin >> f >> s; return pair<T, U>(f, s); }
};
qin in1; // input one
template <typename T> void say(const T x, const char* end = "\n") { cout << x << end; }
void say(const ld x, const char* end = "\n") { cout << setprecision(30) << x << end; }
template <typename T> void say(const vector<T> x, const char* sep = " ", const char* end = "\n") { REP(i, x.size()) { cout << x[i] << (i+1 == i_len ? end : sep); } }
template <typename T> void say(const vector<vector<T>> x, const char* sep = " ", const char* end = "\n") { REP(i, x.size()) { say(x[i], sep, end); } }

// モジュール
// [[LIBRARY]]
// Binary Indexed Tree モジュール (0-indexed)
template <typename T>
class BIT {
private:
    vector<T> val;
public:
    ull n;
    BIT(ull _n) : val(_n+1, 0), n(_n) {}
    BIT(vector<T> arr) : val(arr.size() + 1, 0), n(arr.size()) {
        REP(i, n) add(i, arr[i]);
    }
    // a[i] に v を足す O(logN)
    void add(const ull _i, const T _v) {
        for (ull i = _i + 1; i <= n; i += i & (-i)) {
            val[i] += _v;
        }
    }
    // [0, _i) の和を求める O(logN)
    T get(ull i) const {
        T ans = 0;
        for (; i > 0; i ^= i & (-i)) {
            ans += val[i];
        }
        return ans;
    }
    // [l, r) の和を求める o(N)
    T sum(const ull l, const ull r) {
        return get(r) - get(l);
    }
};

// 処理内容
int main() {
    
    ios::sync_with_stdio(false); // stdioを使うときはコメントアウトすること
    cin.tie(nullptr);            // インタラクティブ問題ではコメントアウトすること

    ll n = in1;
    vector<pair<char, ll>> ca(n*2);
    REP(i, n*2) {
        char c; ll a; cin >> c >> a;
        a--; // 0-indexed
        ca[i] = make_pair(c, a);
    }

    vector<ll> pw(n), pb(n);
    REP(i, n*2) {
        char c; ll a; tie(c, a) = ca[i];
        (c=='W' ? pw : pb)[a] = i;
    }

    // (iw|ib)[i][j] := (w|b)_i以降, (b|w)_j以降が残ってるときに(w|b)_iの左に残っている項の数
    vector<vector<ll>> iw(n, vector<ll>(n+1, INF));
    vector<vector<ll>> ib(n, vector<ll>(n+1, INF));
    DSRNG(i, n-1, 0) {
        ll ws = 0;
        DSRNG(j, pw[i]-1, 0) {
            if (ca[j].first == 'W') {
                if (i < ca[j].second) {
                    ws++;
                }
            }
        }
        iw[i][n] = ws;
        DSRNG(j, n-1, 0) {
            iw[i][j] = iw[i][j+1] + (pw[i] > pb[j]);
        }
    }
    DSRNG(i, n-1, 0) {
        ll bs = 0;
        DSRNG(j, pb[i]-1, 0) {
            if (ca[j].first == 'B') {
                if (i < ca[j].second) {
                    bs++;
                }
            }
        }
        ib[i][n] = bs;
        DSRNG(j, n-1, 0) {
            ib[i][j] = ib[i][j+1] + (pb[i] > pw[j]);
        }
    }
    
    vector<vector<ll>> dp(n+1, vector<ll>(n+1, INF));
    dp[0][0] = 0;
    REP(i, n+1) REP(j, n+1) {
        if (i+1 <= n) {
            chmin(dp[i+1][j], dp[i][j] + iw[i][j]);
        }
        if (j+1 <= n) {
            chmin(dp[i][j+1], dp[i][j] + ib[j][i]);
        }
    }
    say(dp[n][n]);

    
}

Measured memory_bytes: 94336