Problem:
Score : 600 points

Problem Statement

There are 2N balls, N white and N black, arranged in a row. The integers from 1 through N are written on the white balls, one on each ball, and they are also written on the black balls, one on each ball.
The integer written on the i-th ball from the left (1 ≤ i ≤ 2N) is a_i, and the color of this ball is represented by a letter c_i.
c_i = W represents the ball is white; c_i = B represents the ball is black.

Takahashi the human wants to achieve the following objective:

For every pair of integers (i,j) such that 1 ≤ i < j ≤ N, the white ball with i written on it is to the left of the white ball with j written on it.

For every pair of integers (i,j) such that 1 ≤ i < j ≤ N, the black ball with i written on it is to the left of the black ball with j written on it.

In order to achieve this, he can perform the following operation:

Swap two adjacent balls.

Find the minimum number of operations required to achieve the objective.

Constraints

1 ≤ N ≤ 2000

1 ≤ a_i ≤ N

c_i = W or c_i = B.

If i ≠ j, (a_i,c_i) ≠ (a_j,c_j).

Input

Input is given from Standard Input in the following format:

N
c_1 a_1
c_2 a_2
:
c_{2N} a_{2N}

Output

Print the minimum number of operations required to achieve the objective.

Sample Input 1

3
B 1
W 2
B 3
W 1
W 3
B 2

Sample Output 1

4

The objective can be achieved in four operations, for example, as follows:

Swap the black 3 and white 1.

Swap the white 1 and white 2.

Swap the black 3 and white 3.

Swap the black 3 and black 2.

Sample Input 2

4
B 4
W 4
B 3
W 3
B 2
W 2
B 1
W 1

Sample Output 2

18

Sample Input 3

9
W 3
B 1
B 4
W 1
B 5
W 9
W 2
B 6
W 5
B 3
W 8
B 9
W 7
B 2
B 8
W 4
W 6
B 7

Sample Output 3

41

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include<bits/stdc++.h>
#define ll long long
using namespace std;
const int MAXN = 2001;
const int N = 4001;
inline int lowbit(int x) {return x & -x;}
struct BIT
{
    int c[N];
    void update(int x, int v) {for (; x < N; x += lowbit(x)) c[x] += v;}
    int get(int x, int res = 0) {for (; x > 0; x -= lowbit(x)) res += c[x]; return res;}
}T;
int posw[MAXN], posb[MAXN];
int dp[MAXN][MAXN];
int main()
{
    int n; scanf("%d", &n);
    for (int i = 1; i <= 2 * n; i++)
    {
        char c; int a;
        scanf(" %c %d", &c, &a);
        if (c == 'W') posw[a] = i;
        else posb[a] = i;
        T.update(i, 1);
    }
    memset(dp, 0x3f3f3f3f, sizeof(dp));
    dp[0][0] = 0; 
    for (int i = 0; i <= n; i++)
    {
        if (posw[i]) T.update(posw[i], -1);
        for (int j = 0; j <= n; j++) if (i || j)
        {
            if (j) dp[i][j] = min(dp[i][j], dp[i][j - 1] + T.get(posb[j] - 1));
            if (posb[j]) T.update(posb[j], -1);
            if (i) dp[i][j] = min(dp[i][j], dp[i - 1][j] + T.get(posw[i] - 1));
            // printf("dp[%d][%d] = %d\n", i, j, dp[i][j]);
        }
        for (int j = 1; j <= n; j++) T.update(posb[j], 1);
    }
    printf("%d\n", dp[n][n]);
    return 0;
}