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: Time Limit Exceeded)

import java.util.Scanner;

public class Main {
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);

		String S = " " + sc.nextLine() + " ";
		String reg = "BW";

		int count = S.split(reg).length -1;

		while(true) {
			S = replaceByHand(S, reg, "WB");
			count += S.split(reg).length -1;

			if (S.split(reg).length == 1) {
				break;
			}
		}

		System.out.println(count);
	}
	
	private static String replaceByHand(final String original, final String old, final String replace) {
	    int offset = 0;
	    StringBuilder builder = new StringBuilder(original.length());
	    int index;
	    while (0 <= (index = original.indexOf(old, offset))) {
	        builder.append(original, offset, index);
	        builder.append(replace);
	        offset = index + old.length();
	    }
	    builder.append(original, offset, original.length());
	    return builder.toString();
	}
}