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>
using namespace std;
#define MAXN 4000
#define rint register int
inline int read(){int r;int s=0,c;for(;!isdigit(c=getchar());s=c);for(r=c^48;isdigit(c=getchar());(r*=10)+=c^48);return s^45?r:-r;}
int B[MAXN+5][2005], W[MAXN+5][2005], f[2005][2005], w[MAXN+5], b[MAXN+5], n; string S;
int main()
{
	n = read(); for(rint i = 1, a; i <= n<<1; cin >> S, (S[0]=='W'?W:B)[i][a=read()]++, (S[0]=='W'?w:b)[a] = i, i++);
	for(rint i = 1, j; i <= n<<1; i++)
	{
		for(j = n; j; B[i][j] += B[i][j+1], W[i][j] += W[i][j+1], j--);
		for(j = n; j; B[i][j] += B[i-1][j], W[i][j] += W[i-1][j], j--);
	}	memset(f,0x3f,sizeof f), f[0][0] = 0;
	for(rint i = 0, j; i <= n; i++) for(j = 0; j <= n; j++)
		i<n ? f[i+1][j] = min(f[i+1][j],f[i][j]+B[b[i+1]-1][i+1]+W[b[i+1]-1][j+1]) : 0,
		j<n ? f[i][j+1] = min(f[i][j+1],f[i][j]+B[w[j+1]-1][i+1]+W[w[j+1]-1][j+1]) : 0; printf("%d\n",f[n][n]); return 0;
}