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: import sys
  
  input = sys.stdin.readline
  
  N = int(input())
  black = [None] * (N + 1)  # 数字 -> 場所
  white = [None] * (N + 1)
  for i in range(1, 2 * N + 1):
      c, a = input().split()
      a = int(a)
      if c == "W":
          white[a] = i
      else:
          black[a] = i
  
  after = []
  b = 1
  w = 1
  while True:
      if b <= N and w <= N:
          ib = black[b]
          iw = white[w]
          if ib < iw:
              after.append(ib)
              b += 1
          else:
              after.append(iw)
              w += 1
      elif b <= N:
          after.append(black[b])
          b += 1
      elif w <= N:
          after.append(white[w])
          w += 1
      else:
          break
  
  # あとは転倒数を数えて終わり
  
  
  def BIT_update(tree, x):
      L = len(tree)
      while x < L:
          tree[x] += 1
          x += x & (-x)
  
  
  def BIT_sum(tree, x):
      L = len(tree)
      s = 0
      while x:
          s += tree[x]
          x -= x & (-x)
      return s
  
  
  tree = [0] * (2 * N + 10)
  
  answer = 0
  for i, x in enumerate(after):
      answer += i - BIT_sum(tree, x)
      BIT_update(tree, x)
  
  print(answer)
  
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"}]