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;

public class AtCoder
{
    public static void Main()
    {
        string s;
        s = Console.ReadLine().Trim();
        int[] a = new int[s.Length];
        string t = "";
        int w = 0;
        for (int i = 0; i < s.Length; i++)
        {
            t = s.Substring(i, 1);
            if (t == "B") a[i] = 0;
            else
            {
                a[i] = 1;
                w++;
            }
        }
        long x = w * (w + 1) / 2;
        long y = 0;
        long ans = 0;

        for (int i = 0; i < s.Length; i++)
        {
            if(a[i] == 1)
            {
                y += i + 1;
            }
        }
        ans = y - x;



        Console.WriteLine(ans);
    }
}