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: Wrong Answer)

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var sc = bufio.NewScanner(os.Stdin)

func nextInt() int {
	sc.Scan()
	i, e := strconv.Atoi(sc.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func scanStrings(len int) (strings []string) {
	var str string
	for i := 0; i < len; i++ {
		fmt.Scanf("%s", &str)
		strings = append(strings, str)
	}
	return
}

func main() {
	count := 0

	var str string
	fmt.Scanf("%s", &str)
	for {
		if strings.Contains(str, "BW") {
			str2 := strings.Replace(str, "BW", "WB", -1)
			for i := 0; i < len(str2); i++ {
				if str[i] != str2[i] {
					count++
				}
			}
			str = str2
			str = strings.Replace(str, "BBB", "BB", -1)
			str = strings.Replace(str, "WWW", "WW", -1)
		} else {
			break
		}
	}
	fmt.Println(count / 2)
}