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: Time Limit Exceeded)

// inputに入力データ全体が入る
function Main(input) {
  var input = input.replace('\n', '');
  var S = [];
  for (var i = 0; i < input.length; i++){
    S.push(input.substr(i, 1));
  }

  var ans = 0;
  var idx = 0;
  while (idx < S.length - 1) {
    if (S[idx] === 'B' && S[idx + 1] === 'W') {
      S[idx] = 'W';
      S[idx + 1] = 'B';
      ans++;
      // idx = 0;
      idx = (idx === 0)? 0 : idx - 1;
    } else {
      idx++;
    }
  }
  console.log('%d', ans);

}
//*この行以降は編集しないでください（標準入出力から一度に読み込み、Mainを呼び出します）
Main(require("fs").readFileSync("/dev/stdin", "utf8"));