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 C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

// デバッグ実行：Ctrl+F5

using System;

class Program
{
    static void Main(string[] args)
    {
        String line;
        line = Console.ReadLine();
        String S = line;
        int placeofW = -1;
        for(int i = S.Length - 1; i >= 0; i--)
        {
            // 最奥のWを見つける
            if (S[i] == 'W')
            {
                placeofW = i;
                break;
            }
        }
        if (placeofW == -1)
        {
            // 動かせない
            Console.WriteLine("0");
            return;
        }
        int count = 0; //動かした回数
        for(int i = placeofW - 1; i >= 0; i--)
        {
            if (S[i] == 'B')
            {
                count += placeofW - i;
                placeofW--;
            }
        }
        Console.WriteLine(count);
    }
}