Contest: Task: Related: TaskB

Score : $300$ points

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.

- $1 \leq |S| \leq 2\times 10^5$
- $S_i=$
`B`

or`W`

Input is given from Standard Input in the following format:

$S$

Print the maximum possible number of times the operation can be performed.

BBW

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.

BWBWBW

6