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 Julia (0.5.0) (judge verdict: Wrong Answer)

function rank(s)
    rankB = [i for (i, c) = enumerate(s) if c == 'B']
    rankW = [i for (i, c) = enumerate(s) if c == 'W']
    return rankB, rankW
end

function main()
    s = chomp(readline())
    n = length(s)

    rankB, rankW = rank(s)
    nb = length(rankB)
    nw = n - nb

    if length(rankB) < length(rankW)
        rank_given = rankB
        rank_desired = Vector{Int}(nw+1:n)
    else
        rank_given = rankW
        rank_desired = Vector{Int}(1:nw)
    end

    rank_given - rank_desired |> sum |> print
end

main()