output_description: Print the maximum possible number of times the operation can be performed.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      S
attempt: s = list(str(input()))
  sm = [1 if s[i] == "B" else 0 for i in range(len(s))]
  smr = list(reversed(sm))
  t = []
  for i in range(len(s)):
      if smr[i] == 1:
          t.append(i)
  u = [i for i in range(len(t))]
  v = [t[i] - u[i] for i in range(len(t))]
  ans = sum(v)
  print(ans)
  
problem_description: Statement
  
  There are N Reversi pieces arranged in a row. (A _Reversi piece_ is a disc
  with a black side and a white side.) The state of each piece is represented by
  a string S of length N. If S_i=`B`, the i-th piece from the left is showing
  black; If S_i=`W`, the i-th piece from the left is showing white.
  
  Consider performing the following operation:
  
    * Choose i (1 \leq i < N) such that the i-th piece from the left is showing black and the (i+1)-th piece from the left is showing white, then flip both of those pieces. That is, the i-th piece from the left is now showing white and the (i+1)-th piece from the left is now showing black.
  
  Find the maximum possible number of times this operation can be performed.
samples: [{"input": "BBW", "output": "2\n    \n\nThe operation can be performed twice, as follows:\n\n  * Flip the second and third pieces from the left.\n  * Flip the first and second pieces from the left.\n\n* * *"}, {"input": "BWBWBW", "output": "6"}]