// vvvvvvvvvvvv TEMPLATE vvvvvvvvvvvv
#include <bits/stdc++.h>
using namespace std; using ll = long long; using P = pair<ll, ll>;
const ll linf = 1e18; const double eps = 1e-12, pi = acos(-1);
#define FOR(i,a,b) for (ll i=(a),__last_##i=(b);i<__last_##i;i++)
#define RFOR(i,a,b) for (ll i=(b)-1,__last_##i=(a);i>=__last_##i;i--)
#define REP(i,n) FOR(i,0,n)
#define RREP(i,n) RFOR(i,0,n)
#define __GET_MACRO3(_1, _2, _3, NAME, ...) NAME
#define each(i,a) for (auto&& i : a)
#define rep(...) __GET_MACRO3(__VA_ARGS__, FOR, REP)(__VA_ARGS__)
#define rrep(...) __GET_MACRO3(__VA_ARGS__, RFOR, RREP)(__VA_ARGS__)
#define pb push_back
#define eb emplace_back
#define all(a) begin(a),end(a)
#define chmin(x,v) x = min(x, v)
#define chmax(x,v) x = max(x, v)
#define min(x,y) (x < y ? x : y)
#define max(x,y) (x < y ? y : x)
template<typename Head> void out(Head h) { cout << h << endl; } template<typename Head, typename... Tail>void out(Head h, Tail... t) { cout << h << " "; out(t...); }
template<typename T> istream& operator>>(istream& is, vector<T>& v) { each(x,v) is >> x; return is; }
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v) { rep(i,v.size()) { if (i) os << " "; os << v[i]; } return os; }
template<typename T> ostream& operator<<(ostream& os, const vector<string>& v) { rep(i,v.size()) { if (i) os << endl; os << v[i]; } return os; }
template<typename T> ostream& operator<<(ostream& os, const vector<vector<T>>& v) { rep(i,v.size()) { if (i) os << endl; os << v[i]; } return os; }
struct yes_no : std::numpunct<char> { string_type do_truename() const { return "Yes"; } string_type do_falsename() const { return "No"; } };
void solve(); int main() {
  ios::sync_with_stdio(false); cin.tie(0); locale loc(locale(), new yes_no); cout.imbue(loc); cout << fixed << setprecision(10) << boolalpha;
  solve();
}
// ^^^^^^^^^^^^ TEMPLATE ^^^^^^^^^^^^

template <class Monoid>
class SegmentTree {
  using T = typename Monoid::type;
  const int size_, n;
  std::vector<T> data;
  int expand(int m) const { return m <= 1 ? 1 : expand((m + 1) / 2) * 2; }
public:
  SegmentTree() : SegmentTree(0) {}
  SegmentTree(const std::vector<T> &vec) :
    size_(vec.size()), n(expand(size_)), data(n * 2, Monoid::id()) {
    std::copy(begin(vec), end(vec), begin(data) + n);
    for (int i = n - 1; i >= 0; --i) {
      data[i] = Monoid::op(data[i * 2 + 0], data[i * 2 + 1]);
    }
  }
  SegmentTree(const int count, const T &value = Monoid::id()) :
    SegmentTree(std::vector<T>(count, value)) {}
  int size() const { return size_; }
  void update(int pos, const T &value) {
    assert (0 <= pos && pos < size_); // assertion
    data[pos += n] = value;
    while (pos /= 2) {
      data[pos] = Monoid::op(data[pos * 2], data[pos * 2 + 1]);
    }
  }
  T find(int l, int r) const {
    assert (0 <= l && l <= r && r <= size_); // assertion
    l += n; r += n;
    T res1 = Monoid::id(), res2 = Monoid::id();
    while (l != r) {
      if (l % 2) res1 = Monoid::op(res1, data[l++]);
      if (r % 2) res2 = Monoid::op(data[--r], res2);
      l /= 2; r /= 2;
    }
    return Monoid::op(res1, res2);
  }
  using value_type = T;
  using update_type = T;
};

struct Min {
  using type = ll;
  static type id() { return linf; }
  static type op(const type &l, const type &r) { return min(l, r); }
};
struct Max {
  using type = ll;
  static type id() { return -linf; }
  static type op(const type &l, const type &r) { return max(l, r); }
};
struct Sum {
  using type = ll;
  static type id() { return 0; }
  static type op(const type &l, const type &r) { return l + r; }
};

void solve() {
  ll n; cin >> n;
  vector<SegmentTree<Sum>> seg;
  vector<ll> bpos(n), wpos(n);
  rep(i, 2*n) {
    string t; cin >> t;
    ll p; cin >> p; --p;
    if (t == "B") {
      bpos[p] = i;
    }
    else {
      wpos[p] = i;
    }
  }
  vector<ll> seg0(2*n, 1);
  rep(b, n+1) {
    seg.pb(seg0);
    if (b < n) {
      seg0[bpos[b]] = 0;
    }
  }
  vector<vector<ll>> dp(2*n+1, vector<ll>(n+1, linf));
  dp[0][0] = 0;
  rep(i, 2*n) {
    rep(b, n+1) {
      if (dp[i][b] == linf) continue;
      ll w = i - b;
      if (b < n) {
        chmin(dp[i+1][b+1], dp[i][b] + seg[b].find(0, bpos[b]));
      }
      if (w < n) {
        chmin(dp[i+1][b], dp[i][b] + seg[b].find(0, wpos[w]));
      }
    }
    rep(b, n+1) {
      ll w = i - b;
      if (0 <= w && w < n) {
        seg[b].update(wpos[w], 0);
      }
    }
  }
  cout << dp[2*n][n] << endl;
}


Measured memory_bytes: 191232