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;

namespace Main
{
    class Program
    {
        static void Main(string[] args)
        {
            var S = Console.ReadLine();
            int ans = 0;
            int[] Wlist = new int[S.Length+1];
            for(int i = Wlist.Length -2; i >= 0; i--)
            {
                Wlist[i] = S[i] == 'W' ? Wlist[i + 1] + 1 : Wlist[i + 1];
            }
            for(int i=0;i<S.Length-1;i++)
            {
                if (S[i] == 'B')
                    ans += Wlist[i + 1];
            }
            Console.WriteLine(ans);

        }
    }
}