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

-----Sample Output-----
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.

Difficulty: competition

Starter code:

Solutions:
N = int(input())
X = [i for i in range(N+1)]
Y = [[] for _ in range(N)]
B, W = [], []
ans = 0
for i in range(2 * N):
    c, a = input().split()
    a = int(a) - 1
    if c == "B":
        X = [X[i] + 1 if i <= a else X[i] - 1 for i in range(N+1)]
        B.append(a)
        ans += len([b for b in B if b > a])
    else:
        Y[a] = X[:]
        W.append(a)
        ans += len([b for b in W if b > a])

Z = [0] * (N+1)
for y in Y:
    for i in range(N+1):
        Z[i] += y[i]
    for i in range(1, N+1):
        Z[i] = min(Z[i], Z[i-1])

ans += Z[-1]
print(ans)