// InterestingLSY
// codeforces&github: interestingLSY, qq: 2659723130
// 《幽灵公主》是真的好看！
#ifdef intLSY
#include <intlsy/stdc++.h>
#else
#include <bits/stdc++.h>
#endif
#include <bits/c++0x_warning.h>
using namespace std;
#define il inline
#define elif else if

// Type
#define ld double
#define ll long long
#define ull unsigned ll

// Vector
#define vc vector
#define Pb push_back
#define Pf push_front
#define Eb emplace_back
#define All(x) x.begin(),x.end()
#define AllRev(x) x.rbegin(),x.rend()

// Memory
#define Ms(_data) memset(_data,0,sizeof(_data))
#define Msn(_data,_num) memset(_data,_num,sizeof(_data))

// Pair
#define F first
#define S second
#define pii pair<int,int>
#define pli pair<ll,int>
#define pil pair<int,ll>
#define pll pair<ll,ll>

// Loop
#define For(i,j) for( int (i) = 1 ; (i) <= (j) ; ++(i) )
#define For0(i,j) for( int (i) = 0 ; (i) < (j) ; ++(i) )
#define Forx(i,j,k) for( int (i) = (j) ; (i) <= (k) ; ++(i) )
#define Forstep(i,j,k,st) for( int (i) = (j) ; (i) <= (k) ; (i) += (st) )
#define fOR(i,j) for( int (i) = (j) ; (i) >= 1 ; --(i) )
#define fOR0(i,j) for( int (i) = (j)-1 ; (i) >= 0 ; --(i) )
#define fORx(i,j,k) for( int (i) = (k) ; (i) >= (j) ; --(i) )

// Read
struct InputReader{
	static constexpr int MAXSZ = 1048576;
	char buf[MAXSZ]; int p, ed;
	bool detectedEOF = false;
	il InputReader(){ p = MAXSZ; }
	il void Flush(){ p = 0; ed = fread(buf,1,MAXSZ,stdin); }
	il void GetChar( char &c ){
		if(p >= MAXSZ) Flush();
		if( p == ed ){ detectedEOF = true; }
		c = buf[p++];
	}
	il void ReadNum( char &c ){ GetChar(c); while( !isdigit(c) && c != '-' && !detectedEOF ) GetChar(c); }
	il void ReadAlpha( char &c ){ GetChar(c); while( !isalpha(c) && !detectedEOF ) GetChar(c); }
	il void ReadInt( int &ans ){
		ans = 0; int nega = 1;
		char c; ReadNum(c); if(detectedEOF) return;
		if( c == '-' ){
			nega = -1;
			GetChar(c);
		}
		while( isdigit(c) && !detectedEOF ){
			ans = ans*10 + c-'0';
			GetChar(c);
		}
		ans *= nega;
	}
	il void Readll( ll &ans ){
		ans = 0; int nega = 1;
		char c; ReadNum(c); if(detectedEOF) return;
		if( c == '-' ){
			nega = -1;
			GetChar(c);
		}
		while( isdigit(c) && !detectedEOF ){
			ans = ans*10 + c-'0';
			GetChar(c);
		}
		ans *= nega;
	}
	il void ReadString( string &x ){
		x.clear(); char ch; GetChar(ch);
		while( !isdigit(ch) && !isalpha(ch) && ch != '#' && ch != '.' && !detectedEOF ) GetChar(ch); if(detectedEOF) return;
		while( (isdigit(ch) || isalpha(ch) || ch == '#' || ch == '.') && !detectedEOF ){
			x += ch;
			GetChar(ch);
		}
	}
	il void ReadCString( char s[] ){
		int len = 0; char ch; GetChar(ch);
		while( !isdigit(ch) && !isalpha(ch) && ch != '#' && ch != '.' && !detectedEOF ) GetChar(ch); if(detectedEOF) return;
		while( (isdigit(ch) || isalpha(ch) || ch == '#' || ch == '.') && !detectedEOF ){
			s[len++] = ch;
			GetChar(ch);
		}
		s[len] = '\0';
	}
	il void SpecialRead( char &c ){
		GetChar(c); if(detectedEOF) return;
		while( !isdigit(c) && !isalpha(c) && c != '#' && c != '.' && c != '=' && c != 'B' && !detectedEOF ) GetChar(c);
	}
}In;
il void Read( int &x ){ In.ReadInt(x); }
il void Read( int &x, int &y ){ In.ReadInt(x); In.ReadInt(y); }
il void Read( int &x1 , int &x2 , int &x3 ){ In.ReadInt(x1); In.ReadInt(x2); In.ReadInt(x3); }
il void Read( int &x1 , int &x2 , int &x3 , int &x4 ){ In.ReadInt(x1); In.ReadInt(x2); In.ReadInt(x3); In.ReadInt(x4); }
il void Read( ll &x ){ In.Readll(x); }
il void Read( ll &x, ll &y ){ In.Readll(x); In.Readll(y); }
il void Read( ll &x1 , ll &x2 , ll &x3 ){ In.Readll(x1); In.Readll(x2); In.Readll(x3); }
il void Read( ll &x1 , ll &x2 , ll &x3 , ll &x4 ){ In.Readll(x1); In.Readll(x2); In.Readll(x3); In.Readll(x4); }
template<typename T> void Read( T a[] , int st , int ed ){ Forx(i,st,ed) Read(a[i]); }
#define iRead(k) int k; Read(k);
#define iRead2(a,b) iRead(a); iRead(b);
#define iRead3(a,b,c) iRead2(a,b); iRead(c);
#define iRead4(a,b,c,d) iRead2(a,b); iRead2(c,d);
#define lRead(k) ll k; Read(k);
#define lRead2(a,b) lRead(a); lRead(b);
#define lRead3(a,b,c) lRead2(a,b); lRead(c);
#define lRead4(a,b,c,d) lRead2(a,b); lRead2(c,d);

// File
#define Fin(a) freopen(a,"r",stdin)
#define Fout(a) freopen(a,"w",stdout)
il void FILEIO(){
	#ifdef intLSY
		Fin("in.in");
	#endif
}
il void FILEIO( string pname ){
	#ifndef intLSY
		Fin((pname+".in").c_str());
		Fout((pname+".out").c_str());
	#else
		Fin("in.in");
	#endif
}
il void FILEIO_OICONTEST( string pname ){
	Fin((pname+".in").c_str());
	#ifndef intLSY
		Fout((pname+".out").c_str());
	#endif
}
void Printtime(){
	#ifdef intLSY
		double _timeuse = clock()* 1000.0 / CLOCKS_PER_SEC;
		fprintf(stderr,"\n\nTime usage:\n%.0lf ms\n",_timeuse);
	#endif
}
void END(){ Printtime(); exit(0); }
template<typename T>void END( T mes ){ cout << mes << endl; END(); }

// Debug
#define B cerr << "BreakPoint" << endl;
#define O(x) cerr << #x << " " << (x) << endl;
#define o(x) cerr << #x << " " << (x) << "  ";
template<typename T0>
void P( T0 t0 ){
	cerr << t0 << endl;
}
template<typename T0, typename... Ts>
void P( T0 t0 , Ts... ts ){
	cerr << t0 << " ";
	P(ts...);
}
#define Msz(x) cerr << "Sizeof " << #x << " " << sizeof(x)/1024/1024 << " MB" << endl;
template<typename T>void Print( T a[] , int s , int t , char sp = ' ' , char ed = '\n' ){
	if( s > t ) return;
	for( int i = s ; i < t ; i++ )
		cout << a[i] << sp;
	cout << a[t] << ed;
	cout.flush();
}
template<typename T>void Print( T a , int s = 0 , int t = -1 , char sp = ' ' , char ed = '\n' ){
	if( t == -1 ) t = a.size()-1;
	for( int i = s ; i <= t ; i++ )
		cout << a[i] << sp;
	cout << ed;
	cout.flush();
}

// Optimize
#define Max(a,b) ((a)>(b)?(a):(b))
#define Min(a,b) ((a)<(b)?(a):(b))
#define Mymax(a,b) (a) = max((a),(b))
#define Mymin(a,b) (a) = min((a),(b))
#define MyMax(a,b) (a) = Max((a),(b))
#define MyMin(a,b) (a) = Min((a),(b))
#define INF (0x3f3f3f3f)
#define LINF ((long long)(0x3f3f3f3f3f3f3f3f))
#define Sqr(x) ((x)*(x))
#define Lowbit(a) ((a)&(-(a)))
///////////////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////////////
constexpr int MAXN = 2048;

int n;
struct Item{ bool color; int index; };
Item a[MAXN*2];

int poses[2][MAXN];
int cnt[2][MAXN][MAXN*2];
// cnt[c][i][j] 表示有多少个颜色为 c 的 item
// 其编号 <= i 并且位置 >= j
void InitCnt(){
	For(i,n+n) poses[a[i].color][a[i].index] = i;
	For0(col,2){
		auto cnt = ::cnt[col];
		For(i,n+n)
			if( a[i].color == col )
				++cnt[a[i].index][i];
		For(i,n)
			For(j,n+n)
				cnt[i][j] += cnt[i-1][j];
		For(i,n)
			fOR(j,n+n){
				cnt[i][j] += cnt[i][j+1];

				// int t = 0;
				// For(k,n+n)
				// 	if( a[k].color == col and k >= j and a[k].index <= i )
				// 		++t;
				// assert( cnt[i][j] == t );
			}
	}
}

int dp[MAXN][MAXN];
#define NOW dp[use0][use1]
#define KNOW ~NOW
int Dfs( int use0 , int use1 ){
	int totUse = use0 + use1;
	if( totUse == n+n ) return 0;
	if(KNOW) return NOW;
	int ans = INF;
	For0(vCol,2){
		int vColUse = vCol ? use1 : use0;
		if( vColUse == n ) continue;
		int vPos = totUse + 1;
		int initVPos = poses[vCol][vColUse+1];
		int nowVPos = initVPos + cnt[0][use0][initVPos+1] + cnt[1][use1][initVPos+1];
		assert( nowVPos >= vPos );
		int tans = Dfs(use0+(1-vCol),use1+vCol) + nowVPos - vPos;
		MyMin(ans,tans);
	}
	return NOW = ans;
}

int main(){
	FILEIO();
	Msn(dp,-1);

	Read(n);
	For(i,n+n){
		char c; In.ReadAlpha(c);
		a[i].color = c == 'B';
		Read(a[i].index);
	}

	InitCnt();

	int ans = Dfs(0,0);
	cout << ans << endl;

	END();
}


Measured memory_bytes: 82560