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 (1.42.0) (judge verdict: Wrong Answer)

#[allow(dead_code)]

pub struct Scanner {
    idx: usize, 
    buf: Vec<String>,
}

impl Scanner {
    pub fn new<T: std::io::Read>(inf: &mut T) -> Scanner {
        Self {
            idx:0,
            buf: {
                let mut s = String::new();
                inf.read_to_string(&mut s).expect("I/O error");
                s.split_whitespace().map(|x| x.to_owned()).collect()
            },
        }
    }

    pub fn read<T: std::str::FromStr> (&mut self) -> T 
    where
        <T as std::str::FromStr>::Err: std::fmt::Debug,
    {
        if self.empty() {
            panic!("reached the end of input")
        }
        let ret = self.buf[self.idx].parse::<T>().expect("parse error");
        self.idx += 1;
        return ret;
    }

    pub fn empty(&self) -> bool {
        return self.idx >= self.buf.len();
    }
}

fn main() {
    let mut sc = Scanner::new(&mut std::io::stdin());
    let s: Vec<char> = sc.read::<String>().chars().collect();
    let n = s.len();
    let mut ans = 0;
    let mut cnt = 0;

    for i in 0..n {
        if s[i] == 'W' {
            ans += cnt;
        }
        else {
            cnt += 1;
        }
    }

    println!("{}", ans);
}