//marico el que lo lea
#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <stdlib.h>
#include <assert.h>
using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef pair<int, int> ii;

void fastIO() {std::ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);}

#define pb push_back
#define all(obj) obj.begin(), obj.end()
#define ms(obj, val) memset(obj, val, sizeof(obj))
#define ms2(obj, val, sz) memset(obj, val, sizeof(obj[0])*sz)

#define fst first
#define snd second

template<typename T, typename U> inline void mnze(T &x, U y) {if(y < x) x = y;}
template<typename T, typename U> inline void mxze(T &x, U y) {if(x < y) x = y;}

void _scan( int &x ) { scanf("%d",&x); }
void _scan( long long &x ) { scanf("%lld",&x); }
void _scan( double &x ) { scanf("%lf",&x); }
void _scan( char &x ) { scanf(" %c",&x); }
void _scan( char *x ) { scanf("%s",x); }
template<typename T>
void _scan( vector<T> &v ){
	for(int i=0; i<(int)v.size(); i++) _scan(v[i]);
}
void scan() {}
template<typename T, typename... U>
void scan( T& head, U&... tail ) { _scan(head); scan(tail...);}

template<typename T> void _dbg(const char* sdbg, T h)
	{ cerr<<sdbg<<"="<<h<<"\n"; }
template<typename T, typename... U> void _dbg(const char* sdbg, T h, U... t) {
	while(*sdbg != ',')cerr<<*sdbg++;
	cerr<<"="<<h<<","; _dbg(sdbg+1, t...);
}

#ifdef LOCAL
#define debug(...) _dbg(#__VA_ARGS__, __VA_ARGS__)
#define debugv(x) {{cerr<<#x<<" = "; for(auto _i:x) cerr<<_i<<", ";cerr<<"\n";}}
#define debuga(x, sz) {{cerr<<#x<<" = ";for(int _i=0;_i<sz;_i++)cerr<<x[_i]<<", ";cerr<<"\n";}}
#else
#define debug(...) (__VA_ARGS__)
#define debugv(x)
#define debuga(x, sz)
#define cerr if(0)cout
#endif

ll MOD = 1e9+7;

inline ll Msum(ll x){return x;}
template<typename... Rest>
inline ll Msum(ll x, Rest... rest){ return (x + Msum(rest...)) % MOD; }

inline ll Mprod(ll x){return x;}
template<typename... Rest>
inline ll Mprod(ll x, Rest... rest){ return x * Mprod(rest...) % MOD; }

inline ll Mnorm(ll x){ return (x%MOD + MOD) %MOD; }

inline ll Msq(ll x){ return (x*x) %MOD; }
inline ll Mpot(ll x, ll e){
	if(!e) return 1;
	return Msq(Mpot(x, e>>1)) * (e & 1 ? x : 1)%MOD;
}
////////////////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////////////////

const int MAXN = 2e3+3;

int n;
int a[2*MAXN], c[2*MAXN];
int pos[2][MAXN];
int p[2][MAXN][2*MAXN];

int dp[MAXN][MAXN];
int f(int ib, int iw){
	if(ib==n && iw == n) return 0;
	int &ret = dp[ib][iw];
	if(ret != -1) return ret;
	ret = 1e9;
	if(ib < n){
		int j = pos[0][ib+1];
		int nret = f(ib+1, iw);
		if(j){
			nret += p[0][ib][j-1];
			nret += p[1][iw][j-1];
		}
		mnze(ret, nret);
	}
	if(iw < n){
		int j = pos[1][iw+1];
		int nret = f(ib, iw+1);
		if(j){
			nret += p[0][ib][j-1];
			nret += p[1][iw][j-1];
		}
		mnze(ret, nret);
	}
	return ret;
}

int main(){
	scan(n);
	for(int i=0; i<2*n; i++){
		char aux[3];
		scan(aux, a[i]);
		c[i] = (aux[0] == 'W'); // 0=B   1=W
		pos[c[i]][a[i]] = i;
	}
	for(int x=0; x<n+1; x++){
		if(a[0] > x) p[c[0]][x][0]++;
		for(int i=1; i<2*n; i++){
			p[0][x][i] = p[0][x][i-1];
			p[1][x][i] = p[1][x][i-1];
			if(a[i] > x) p[c[i]][x][i]++;
		}
	}
	ms(dp, -1);
	printf("%d\n", f(0, 0));
}


Measured memory_bytes: 78848