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 Lua (Lua 5.3.5) (judge verdict: Accepted)

local w = string.byte("W")

local s = io.read()
local s_len = s:len()
local b_count = 0
local count = 0
local last_w_pos = 0
for i = 1, s_len do
	local is_w = s:byte(i) == w
	if b_count ~= 0 and is_w then
		count = count + b_count
		last_w_pos = last_w_pos + 1
	elseif is_w then
		last_w_pos = last_w_pos + 1
	else
		b_count = b_count + 1
	end
end

print(count)