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 Ruby (2.7.1) (judge verdict: Runtime Error)

s = gets.chomp!.split("")
N = s.length
count = 0
left_w_count = 0
num_of_w = s.count("W")

loop do
  left_w_count = s.index("B")
  (left_w_count..N - 1).each do |i|
    # pp "DEBUG:i=#{i}"
    if s[i] == "B" and s[i + 1] == "W"
      s[i] = "W"
      s[i + 1] = "B"
      count += 1
      flag = 0
      #   pp "DEBUG:s=#{s}"
      break
    end
  end
  break if left_w_count == num_of_w
end

puts count