Problem:
Score : 300 points

Problem 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.

Constraints

1 \leq |S| \leq 2\times 10^5

S_i=B or W

Input

Input is given from Standard Input in the following format:

S

Output

Print the maximum possible number of times the operation can be performed.

Sample Input 1

BBW

Sample Output 1

2

The operation can be performed twice, as follows:

Flip the second and third pieces from the left.

Flip the first and second pieces from the left.

Sample Input 2

BWBWBW

Sample Output 2

6

Solution in Scala (2.13.1) (judge verdict: Wrong Answer)

object Main extends App {
  @scala.annotation.tailrec
  def loop(chars: Seq[Char], step: Int, acc: Int): Int = chars match {
    case Seq() => acc
    case 'W' :: xs =>
      loop(xs, step, acc + step)
    case _ =>
      val (bs, xs) = chars.span(_ != 'W')
      loop(xs, bs.length + step, acc)
  }

  val sc = new java.util.Scanner(System.in)
  println(loop(sc.next().toCharArray.toList, 0, 0))
}