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 PHP (5.6.30) (judge verdict: Time Limit Exceeded)

<?php
$S = str_split(trim(fgets(STDIN)));
$i=0;
$ans = 0;
while($i<count($S)-1){
  if($S[$i]=='B' && $S[$i+1]=='W'){
    $S[$i]='W';
    $S[$i+1]='B';
 
    $i = $i-2 ;
    if($i<0){
      $i=0;
    }    
  $ans++;
  }else{
  $i++;
  }
}
echo $ans;