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;

namespace agc029a
{
    class Program
    {
        static void Main(string[] args)
        {
            string S = Console.ReadLine();
            char[] charS = S.ToCharArray();
            int N = S.Length;
            int count = 0;
            for(int i = N -2; i >= 0; i--){
                if(charS[i]=='B'){
                    int j = i;
                    while(j<N){
                        if(j==N-1){
                            break;
                        }else if(charS[j+1]=='B'){
                            break;
                        }else{
                            charS[j] = 'W';
                            charS[j+1] = 'B';
                            count++;
                            j++;
                        }
                    }
                }
            }
            Console.WriteLine(count);
        }
    }
}