#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")

#include <bits/stdc++.h>
using namespace std;

using PII = pair<int, int>;
using TI3 = tuple<int, int, int>;
using LL = long long;
using ULL = unsigned long long;
using MAT = array<array<LL, 2>, 2>;

template<class T> constexpr inline T begin(const T&) {return 0;}
template<class T> constexpr inline T end(const T& x) {return x;}

#define PN() putchar('\n')
#define PB push_back
#define EB emplace_back
#define GET_MACRO(_1,_2,_3,_4,NAME,...) NAME
#define CHECK(a,b) static_assert(std::is_same<decltype(a), decltype(b)>::value, "REP diff types");

#define REPPP(i,s,c,t) for(i; ((s) and (c)) or (putchar("\n "[c]) and (c)); (t))
#define REPP2(i,n) REPPP(auto i=begin(n),i==begin(n),i!=end(n),++i)
#define REPP3(i,m,n) CHECK(m,n) REPPP(auto i=(m),i==(m),i<(n),++i)
#define REPP4(i,m,n,s) CHECK(m,n) REPPP(auto i=(m),i==(m),((s)>0 and i<(n)) or ((s)<0 and i>(n)),i+=(s))
#define REPP(...) GET_MACRO(__VA_ARGS__, REPP4, REPP3, REPP2)(__VA_ARGS__)

#define REP2(i,n) for(auto i=begin(n);i!=end(n);++i)
#define REP3(i,m,n) CHECK(m,n) for(auto i=(m);i<(n);++i)
#define REP4(i,m,n,s) CHECK(m,n) for(auto i=(m);((s)>0 and i<(n)) or ((s)<0 and i>(n));i+=(s))
#define REP(...) GET_MACRO(__VA_ARGS__, REP4, REP3, REP2)(__VA_ARGS__)

template<class T> void _read(T &x) {cin>>x;}
void _read(size_t &x) {scanf("%zu", &x);}
void _read(int &x) {scanf("%d", &x);}
void _read(LL &x) {scanf("%lld", &x);}
void _read(ULL &x) {scanf("%llu", &x);}
void _read(double &x) {scanf("%lf", &x);}
void _read(char &x) {scanf(" %c", &x);}
void _read(char *x) {scanf("%s", x);}
void read() {}
template<class T, class... U>
void read(T& head, U&... tail) {
    _read(head);
    read(tail...);
}

template<class T> void _print(const T &x) {cout << x;}
void _print(const size_t &x) {printf("%zu", x);}
void _print(const int &x) {printf("%d", x);}
void _print(const LL &x) {printf("%lld", x);}
void _print(const ULL &x) {printf("%llu", x);}
void _print(const double &x) {printf("%.16lf",x);}
void _print(const char &x) {putchar(x);}
void _print(const char *x) {printf("%s",x);}
template<class T> void _print(const vector<T> &x) {
    for(auto i = x.begin(); i != x.end(); _print(*i++)) {
        if (i != x.cbegin()) putchar(' ');
    }
}
void print() {}
template<class T, class... U> void print(const T& head, const U&... tail) {
    _print(head);
    putchar(sizeof...(tail) ? ' ' : '\n');
    print(tail...);
}

#define LLINF 0x3f3f3f3f3f3f3f3f
#define INF 0x3f3f3f3f

const int MOD = 1000000007;
#define MAX_N 4005

int bit[MAX_N + 1];
struct Bit {
public:
    Bit(int _n): n(_n) {}
    void reset() {
        memset(bit, 0, sizeof(bit));
    }
    int sum(int i) {
        int s = 0;
        while (i > 0) {
            s += bit[i];
            i -= i & -i;
        }
        return s;
    }

    void add(int i, int x) {
        while (i <= n) {
            bit[i] += x;
            i += i & -i;
        }
    }
private:
    int n;
};

int c[MAX_N], a[MAX_N];
int cost[2][MAX_N][MAX_N];
int dp[MAX_N][MAX_N];
char tmp[100];

int main()
{
    int n;
    read(n);
    REP(i, 2 * n) {
        read(tmp);
        c[i] = int(tmp[0] != 'B');
        read(a[i]);
    }
    Bit b = Bit(2 * n);
    REP(k, 2) {
        REP(i, n + 1) {
            b.reset();
            REP(j, 2 * n - 1, -1, -1) {
                if(c[j] == k) {
                    cost[k][i][a[j]] = b.sum(a[j]);
                    b.add(a[j], 1);
                } else if(a[j] <= i) {
                    b.add(1, 1);
                }
            }
        }
    }

    REP(i, n + 1) {
        REP(j, n + 1) {
            if(i == 0 and j == 0) {
                continue;
            }
            int c1 = INT_MAX, c2 = INT_MAX;
            if(i > 0) {
                c1 = dp[i-1][j] + cost[0][j][i];
            }
            if(j > 0) {
                c2 = dp[i][j-1] + cost[1][i][j];
            }
            dp[i][j] = min(c1, c2);
        }
    }
    print(dp[n][n]);
    return 0;
}


Measured memory_bytes: 98048