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 Rust (1.15.1) (judge verdict: Accepted)

use std::io::prelude::*;
fn main() {
    let mut s = String::new();
    let stdin = std::io::stdin();
    stdin.lock().read_line(&mut s).expect("line");
    let stones: Vec<Stone> = (&s)
        .trim()
        .chars()
        .map(|c| Stone::from_char(c).expect("fromchar error"))
        .collect();
    let len = stones.len();
    let mut ans = 0;
    let mut blacks = 0;
    for (i, &s) in stones.iter().enumerate() {
        if s == Stone::B {
            blacks += 1;
            ans += len;
            ans -= i + blacks;
        }
    }
    println!("{}", ans);
}

#[derive(Debug, Clone, Copy, Eq, PartialEq)]
enum Stone {
    B,
    W,
}

impl Stone {
    fn from_char(c: char) -> Option<Self> {
        match c {
            'B' => Some(Stone::B),
            'W' => Some(Stone::W),
            _ => None,
        }
    }
}