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: Runtime Error)

const main = input => {
  const stones = [ ...input ]
  let counter = 0

  while (stones.some((stone, i, a) => {
    if (i < a.length - 1 && stone == "B" && a[i + 1] == "W") {
      [ a[i], a[i + 1] ] = [ a[i + 1], a[i] ]
      return true
    }
  })) {
    counter ++
  }

  console.log(counter)
}

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