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: Wrong Answer)

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<char> = s.chars().collect();
    let len = stones.len();
    let mut ans = 0;
    let mut blacks = 0;
    for (i, &s) in stones.iter().enumerate() {
        if s == '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(Self::B),
//             'W' => Some(Self::W),
//             _ => None,
//         }
//     }
// }