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 JavaScript (node.js v5.12) (judge verdict: Wrong Answer)

'use strict'

const main = (INPUT) => {
  let S = INPUT.split('\n')[0].split('')
  let ans = 0
  for (let i = S.join('').indexOf('BW'); i < S.length - 1;) {
    S[i] = 'W'
    S[i + 1] = 'B'
    ans++
    i = S.join('').indexOf('BW')
    if (i == -1) {
      break
    }
  }
  console.log(ans)
}

main(require('fs').readFileSync('/dev/stdin', 'utf8'))