#include<bits/stdc++.h>
using namespace std;
const int MAXN = 4009;
int a[MAXN]; char c[MAXN];
int wpos[MAXN], bpos[MAXN];
int wchoto[MAXN][MAXN], bchoto[MAXN][MAXN];

int dp[MAXN][MAXN];

int main() {
    int n;
    cin >> n;
    for (int i = 1; i <= n+n; i++) {
        cin >> c[i] >> a[i];
        if (c[i]=='W') wpos[a[i]] = i;
        else bpos[a[i]] = i;
    }

    for (int i = n+n; i > 0; i--) {
        for (int j = 1; j <= n; j++) {
            wchoto[i][j] = wchoto[i+1][j];
            wchoto[i][j] += c[i]=='W' && a[i] <= j;

            bchoto[i][j] = bchoto[i+1][j];
            bchoto[i][j] += c[i]=='B' && a[i] <= j;
        }
    }

    for (int w = 1; w <= n; w++) {
        dp[w][0] = dp[w-1][0]+wchoto[wpos[w]][w-1];
    }
    for (int b = 1; b <= n; b++) {
        dp[0][b] = dp[0][b-1]+bchoto[bpos[b]][b-1];
    }
    for (int w = 1; w <= n; w++) {
        for (int b = 1; b <= n; b++) {
            dp[w][b] = min( dp[w-1][b]+wchoto[wpos[w]][w-1]+bchoto[wpos[w]][b],
                            dp[w][b-1]+bchoto[bpos[b]][b-1]+wchoto[bpos[b]][w]);
        }
    }

    cout << dp[n][n] << endl;


    return 0;
}
