output_description: Print the minimum number of operations required to achieve the objective.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
      c_1 a_1
      c_2 a_2
      :
      c_{2N} a_{2N}
attempt: N = int(input())
  balls = []
  for i in range(2 * N):
      c, a = input().split()
      balls += [(c, int(a))]
  
  # numB[i][b]: 初期状態において、i番目の玉より右にある、1～bが書かれた黒玉の数
  numB = [[0] * (N + 1) for i in range(2 * N)]
  numW = [[0] * (N + 1) for i in range(2 * N)]
  for i in reversed(range(1, 2 * N)):
      c, a = balls[i]
      numB[i - 1] = numB[i].copy()
      numW[i - 1] = numW[i].copy()
      if c == "B":
          for b in range(a, N + 1):
              numB[i - 1][b] += 1
      else:
          for w in range(a, N + 1):
              numW[i - 1][w] += 1
  
  # costB[b][w]: 黒玉1～b、白玉1～wのうち、初期状態において、黒玉[b+1]より右にある玉の数
  costB = [[0] * (N + 1) for i in range(N + 1)]
  costW = [[0] * (N + 1) for i in range(N + 1)]
  for i in reversed(range(2 * N)):
      c, a = balls[i]
      if c == "B":
          cost = numB[i][a - 1]
          for w in range(N + 1):
              costB[a - 1][w] = cost + numW[i][w]
      else:
          cost = numW[i][a - 1]
          for b in range(N + 1):
              costW[b][a - 1] = cost + numB[i][b]
  
  dp = [[float("inf")] * (N + 1) for i in range(N + 1)]
  dp[0][0] = 0
  for b in range(N + 1):
      for w in range(N + 1):
          if b > 0:
              dp[b][w] = min(dp[b][w], dp[b - 1][w] + costB[b - 1][w])
          if w > 0:
              dp[b][w] = min(dp[b][w], dp[b][w - 1] + costW[b][w - 1])
  
  print(dp[N][N])
  
problem_description: 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.
samples: [{"input": "3\n    B 1\n    W 2\n    B 3\n    W 1\n    W 3\n    B 2", "output": "4\n    \n\nThe objective can be achieved in four operations, for example, as follows:\n\n  * Swap the black 3 and white 1.\n  * Swap the white 1 and white 2.\n  * Swap the black 3 and white 3.\n  * Swap the black 3 and black 2.\n\n* * *"}, {"input": "4\n    B 4\n    W 4\n    B 3\n    W 3\n    B 2\n    W 2\n    B 1\n    W 1", "output": "18\n    \n\n* * *"}, {"input": "9\n    W 3\n    B 1\n    B 4\n    W 1\n    B 5\n    W 9\n    W 2\n    B 6\n    W 5\n    B 3\n    W 8\n    B 9\n    W 7\n    B 2\n    B 8\n    W 4\n    W 6\n    B 7", "output": "41"}]