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 Perl (v5.18.2) (judge verdict: Accepted)

#!/usr/bin/perl

use strict;
use warnings;
use 5.010;

use Data::Dumper;

my @INPUT;
while (<>) {
    chomp;
    push @INPUT, $_;
}

my @S = split "", shift @INPUT;

my @B;
for (my $i = 0; $i < @S; $i++) {
    if ($S[$i] eq 'B') {
        push @B, $i;
    }
}

my $ans = 0;
for (my $i = 0; $i < @B; $i++) {
    $ans += $#S - $B[$i] - ($#B - $i)
}

say $ans;