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)

using System;
using System.Collections.Generic;

namespace ConsoleApplication1
{

    class Program
    {

        static void Main(string[] args)
        {

            string input = Console.ReadLine();

            char[] str = input.ToCharArray();

            int s = str.Length - 1;
            int count = 0;
            int n = str.Length;

            if(str[n - 1] == 'B')
            {               
                for(int i = n - 1; i >= 0; i--)
                {
                    if(str[i] == 'W')
                    {
                        s = i;
                        break;
                    }
                }
            }

            int k = 0;

            for(int i = s; i >= 0; i--)
            {
                if(str[i] == 'B')
                {
                    str[i] = 'W';
                    str[s] = 'B';
                    count += k;
                    s--;
                }
                else
                {
                    k++;
                }

            }


            Console.WriteLine(count);

        }
    }
}