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 Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)

import java.io.InputStream;
import java.io.PrintStream;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;

public class Main {
	InputStream in = System.in;
	PrintStream out = System.out;


	public void _main(String[] args) {
		Scanner sc = new Scanner(in);
		
		String S = sc.next();
		
		List<List<Integer>> a = new ArrayList<>();
		for(int i=0;i<S.length();i++) {
			a.add(Arrays.asList(i, S.charAt(i) == 'W' ? 0 : 1));
		}
		
		a.sort((o1, o2) -> Integer.compare(o1.get(1), o2.get(1)));
		
		int ans = 0;
		for(int i=0;i<S.length();i++) {
			List<Integer> x = a.get(i);
			if(x.get(1) == 0) {
				ans += Math.abs(i-x.get(0));
			}
		}
		out.println(ans);
		
		
	}

	public static void main(String[] args) {
		new Main()._main(args);
	}

}