﻿ AGC029 A - Irreversible operation - Atcoder

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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