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 copy
  
  N = int(input())
  c = []
  for i in range(2 * N):
      a, b = input().split()
      c.append((a, int(b)))
  mo = copy.deepcopy(c)
  ans = 0
  for i in range(2, N + 1):
      d = c.index(("B", i))
      e = c.index(("B", i - 1))
      if e - d >= 0:
          ans += e - d
          c.pop(e)
          c.insert(d, ("B", i - 1))
  
  for i in range(2, N + 1):
      d = c.index(("W", i))
      e = c.index(("W", i - 1))
      if e - d >= 0:
          ans += e - d
          c.pop(e)
          c.insert(d, ("W", i - 1))
  
  gyaku = 0
  for i in range(2, N + 1):
      d = mo.index(("W", i))
      e = mo.index(("W", i - 1))
      if e - d >= 0:
          gyaku += e - d
          mo.pop(e)
          mo.insert(d, ("W", i - 1))
  for i in range(2, N + 1):
      d = mo.index(("B", i))
      e = mo.index(("B", i - 1))
      if e - d >= 0:
          gyaku += e - d
          mo.pop(e)
          mo.insert(d, ("B", i - 1))
  print(min(ans, gyaku))
  
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"}]