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


#define li          long long int
#define rep(i,to)   for(li i=0;i<((li)(to));i++)
#define repp(i,start,to)    for(li i=(li)(start);i<((li)(to));i++)
#define pb          push_back
#define sz(v)       ((li)(v).size())
#define bgn(v)      ((v).begin())
#define eend(v)     ((v).end())
#define allof(v)    (v).begin(), (v).end()
#define dodp(v,n)       memset(v,(li)n,sizeof(v))
#define bit(n)      (1ll<<(li)(n))
#define mp(a,b)     make_pair(a,b)
#define rin rep(i,n)
#define EPS 1e-12
#define ETOL 1e-8
#define MOD 1000000007
typedef pair<li, li> PI;

#define INF bit(60)

#define DBGP 1


#define idp if(DBGP)
#define F first
#define S second
#define p2(a,b)     idp cout<<a<<"\t"<<b<<endl
#define p3(a,b,c)       idp cout<<a<<"\t"<<b<<"\t"<<c<<endl
#define p4(a,b,c,d)     idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<endl
#define p5(a,b,c,d,e)       idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<endl
#define p6(a,b,c,d,e,f)     idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<endl
#define p7(a,b,c,d,e,f,g)       idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<endl
#define p8(a,b,c,d,e,f,g,h)     idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<"\t"<<h<<endl
#define p9(a,b,c,d,e,f,g,h,i)       idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<"\t"<<h<<"\t"<<i<<endl
#define p10(a,b,c,d,e,f,g,h,i,j)        idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<"\t"<<h<<"\t"<<i<<"\t"<<j<<endl
#define foreach(it,v)   for(__typeof((v).begin()) it=(v).begin(); it!=(v).end(); ++it)
#define p2p(x)      idp p2((x).F, (x).S)
#define dump(x,n)   idp{rep(i,n){cout<<x[i]<<" ";}puts("");}
#define dump2(x,n)  idp{rep(i,n){cout<<"["<<x[i].F<<" , "<<x[i].S<<"] ";}puts("");}
#define dumpi(x)    idp{foreach(it, x){cout<<(*it)<<" ";}puts("");}
#define dumpi2(x)   idp{foreach(it, x){cout<<"["<<(it)->F<<" , "<<(it)->S<<"] ";}puts("");}

#define read2d(a,w,h)   rep(i,h)rep(j,w)cin>>a[i][j]
#define dump2d(a,w,h)   rep(i,h){rep(j,w)cout<<a[i][j]<<" ";puts("");}

typedef pair<li, li> PI;


li n;
li b[5050];
li w[5050];

li dp[5050][5050];

// cnt_b[i][j]: i番目までで、ｊ以下の数が何個あるか
li cnt_b[5050][5050];
li cnt_w[5050][5050];

// i:b, j:w
inline li calc(li pos, li i, li j) {
    return (cnt_b[pos][n] - cnt_b[pos][i]) + (cnt_w[pos][n] - cnt_w[pos][j]);
}

int main() {
    cin >> n;
    rep(i, 2 * n) {
        string s;
        li a;
        cin >> s >> a;
        if (s == "W") {
            w[a] = i;
            rep(j, n + 1) {
                cnt_w[i + 1][j] = cnt_w[i][j] + (j >= a ? 1 : 0);
            }
            rep(j, n + 1) {
                cnt_b[i + 1][j] = cnt_b[i][j];
            }
        } else {
            b[a] = i;
            rep(j, n + 1) {
                cnt_w[i + 1][j] = cnt_w[i][j];
            }
            rep(j, n + 1) {
                cnt_b[i + 1][j] = cnt_b[i][j] + (j >= a ? 1 : 0);
            }
        }
    }

    dp[0][0] = 0;
    rep(i, n) {
        dp[0][i + 1] = dp[0][i] + calc(w[i + 1], 0, i + 1);
    }
    repp(i, 1, n + 1) {
        dp[i][0] = dp[i - 1][0] + calc(b[i], i, 0);
        repp(j, 1, n + 1) {
            dp[i][j] = min(dp[i - 1][j] + calc(b[i], i, j),
                           dp[i][j - 1] + calc(w[j], i, j));
        }
    }

    cout << dp[n][n] << endl;
    return 0;
}

Measured memory_bytes: 400768