Problem: 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.

Submission attempts (in order):
from sys import stdin

def main():
    A = list(map(int,stdin.readline().replace('B', '0').replace('W', '1')))

    A=[0,0,1]
    wc = A.count(1)
    indices = [i for i, x in enumerate(A) if x == 1]
    if wc > 0:
      print(sum(indices) - 1*wc*(wc-1) //2 )
    else:
      print(sum(indices))

input = lambda: stdin.readline()
main()


# -*- coding: utf-8 -*-

N = int(input())
A = list(map(int, input().split()))
A.sort(reverse=True)

pairCounter=0
for i in range(N):
  if A[i] < 0:
    continue
  for b in range(9):
    pair=2**b - A[i]
    if pair < 1:
      continue
    
    if A.count(pair) >0:
      A[i] = -1
      A[A.index(pair)] = -1
      pairCounter+=1
      break

print(pairCounter)

from sys import stdin

def main():
    bw=stdin.readline().replace('B', '0').replace('W', '1')
    A=list(map(int,bw))
    #A = list(map(int,stdin.readline().replace('B', '0').replace('W', '1')))

    A=[0,0,1]
    wc = A.count(1)
    indices = [i for i, x in enumerate(A) if x == 1]
    if wc > 0:
      print(sum(indices) - 1*wc*(wc-1) //2 )
    else:
      print(sum(indices))

input = lambda: stdin.readline()
main()


from sys import stdin
def main():
    A = list(map(int,stdin.readline().replace('B', '0').replace('W', '1')))

    wc = A.count(1)
    indices = [i for i, x in enumerate(A) if x == 1]
    if wc > 0:
      print(sum(indices) - 1*wc*(wc-1) //2 )
    else:
      print(sum(indices))

input = lambda: stdin.readline()
main()


from sys import stdin

def main():
    A = list(map(int,stdin.readline().replace('B', '0').replace('W', '1')))

    wc = A.count(1)
    indices = [i for i, x in enumerate(A) if x == 1]
    if wc > 0:
      print(sum(indices) - 1*wc*(wc-1) //2 )
    else:
      print(sum(indices))

input = lambda: stdin.readline()
main()


from sys import stdin
def main():
    A = list(map(int,stdin.readline().replace('B', '0').replace('W', '1')))

    wc = A.count(1)
    indices = [i for i, x in enumerate(A) if x == 1]
    print(sum(indices) - 1*wc*(wc-1) //2 )

input = lambda: stdin.readline()
main()


from sys import stdin

def main():
    A=stdin.readline()
    #A = list(map(int,stdin.readline().replace('B', '0').replace('W', '1')))

    wc = A.count('W')
    indices = [i for i, x in enumerate(A) if x == 'W']
    if wc > 0:
      print(sum(indices) - 1*wc*(wc-1) //2 )
    else:
      print(sum(indices))

input = lambda: stdin.readline()
main()


Verdicts: Runtime Error

Runtime Error

Runtime Error

Runtime Error

Runtime Error

Runtime Error

Accepted