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)

use v5.18;
use warnings;
use Data::Dumper;
use List::Util qw( sum );
sub read_line { chomp(my $s = <>); $s }
sub splited_read_line { split / /, read_line }

my @chars = split //, read_line;
say solve(\@chars);

sub solve {
  my $chars = shift;
  my $first_cost =
    sum map { $_->[0] }
      grep { $_->[1] eq 'B' }
      map { [$_, $chars->[$_] ] } 0 .. $#$chars;
  my @moved_chars = sort { $b cmp $a } @$chars;
  my $last_cost =
    sum map { $_->[0] }
      grep { $_->[1] eq 'B' }
      map { [$_, $moved_chars[$_] ] } 0 .. $#moved_chars;
  $last_cost - $first_cost;
}