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: Time Limit Exceeded)

using System;

class keisan
{
  static void Main()
  {
    //0がblackで1がwhite
//    int[] isi = new int[5]{0, 1, 1, 0, 0};
    /*
    5個のとき6が最大
    i個の要素があるとき、a * b(a + b = i, aとbは自然数)が最大の交換回数
    したがってa * (i - a) (a = 1, 2, ..., i - 1)
    したがってint(i / 2) * (i - int(i / 2))が答え
    */
    
    string str = Console.ReadLine();
    
    char[] c = new char[str.Length];
    for (int i = 0; i < str.Length; i++)
    {
      c[i] = str[i];
    }

    bool check = false;
    int kaisuu = 0;
    while(check == false)
    {
      check = true;
      for (int i = 0; i < str.Length - 1; i++)
      {
        if (c[i] == 'B' && c[i + 1] == 'W')
        {
          c[i] = 'W';
          c[i + 1] = 'B';
          kaisuu++;
          check = false;
        }
      }
    }
  	System.Console.WriteLine(kaisuu);
  }
}