#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
typedef long long ll;
inline int read(){
	int Res=0,f=1;char ch=getchar();
	while (ch>'9'||ch<'0') f=(ch=='-'?-f:f),ch=getchar();
	while (ch>='0'&&ch<='9') Res=Res*10+ch-'0',ch=getchar();
	return Res*f;
}
int N;
int A[4005],B[2005],W[2005],Col[4005];
int F[2005][2005],NB[4005][2005],NW[4005][2005];
int main()
{
	N=read();
	for (int i=1;i<=2*N;i++){
		char S[5];scanf("%s",S);Col[i]=S[0]=='B'?1:0;A[i]=read();
		if (S[0]=='B') B[A[i]]=i;else W[A[i]]=i;
	}
	for (int i=1;i<=2*N;i++){
		for (int j=0;j<=N;j++){
			NB[i][j]=NB[i-1][j]+(A[i-1]>j&&Col[i-1]==1);
			NW[i][j]=NW[i-1][j]+(A[i-1]>j&&Col[i-1]==0);
		}
	}
	memset(F,63,sizeof F);
	F[0][0]=0;
	for (int i=0;i<=N;i++){
		for (int j=0;j<=N;j++){
			if (i) F[i][j]=min(F[i][j],F[i-1][j]+NB[B[i]][i-1]+NW[B[i]][j]);
			if (j) F[i][j]=min(F[i][j],F[i][j-1]+NB[W[j]][i]+NW[W[j]][j-1]);
		}
	}
	printf("%d\n",F[N][N]);
	return 0;
}