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 Go (1.6) (judge verdict: Accepted)

package main

import (
	"bufio"
	"fmt"
	"os"
)

var sc = bufio.NewScanner(os.Stdin)

const lim = 2e5 + 10

func rb() []byte {
	sc.Scan()
	return sc.Bytes()
}

func main() {
	sc.Split(bufio.ScanWords)
	sc.Buffer(make([]byte, lim), lim)
	s := rb()

	ctW := 0
	for _, ss := range s {
		if ss == 'W' {
			ctW++
		}
	}

	bIdx := 0
	for i := 0; i < ctW; i++ {
		if s[i] == 'B' {
			bIdx += i
		}
	}
	wIdx := 0
	for i := ctW; i < len(s); i++ {
		if s[i] == 'W' {
			wIdx += i
		}
	}

	fmt.Println(wIdx - bIdx)
}