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())
  ca = [
      [0 if c == "B" else 1 if c == "W" else (int(c) - 1) for c in input().split()]
      for _ in range(N * 2)
  ]
  
  Bi = [N * 10] * (N + 1)
  Wi = [N * 10] * (N + 1)
  BWi = [[N * 10] * (N + 1) for _ in [0] * 2]
  for i, row in enumerate(ca):
      c, a = row
      BWi[c][a] = i
  ans = 0
  BWc = [0, 0]
  while BWc[0] < N and BWc[1] < N:
      tmp1b = min(BWi[0][BWc[0] + 1 :] + [N * 10])
      tmp1w = min(BWi[1][BWc[1] + 1 :] + [N * 10])
      tc = int(tmp1b > tmp1w)
      tmp1 = min(tmp1b, tmp1w)
      tmp2 = BWi[tc].index(tmp1)
      tmp3 = BWi[tc][BWc[tc] : tmp2]
      tmp0 = min(tmp3) if tmp3 else 0
      if tmp0 > tmp1:
          ans += tmp0 - tmp1
          ca[tmp1 : tmp0 + 1] = [ca[tmp0]] + ca[tmp1:tmp0]
          BWc[tc] -= int(ca[tmp0][1] != BWc[tc])
          for j, row in enumerate(ca[tmp1 : tmp0 + 1], tmp1):
              c, a = row
              BWi[c][a] = j
      BWc[tc] += 1
  print(ans)
  
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"}]