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;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp6
{
    class Program
    {
        // black
        public static char b = 'B';
        // white
        public static char w = 'W';

        /// <summary>
        /// program
        /// </summary>
        /// <param name="args"></param>
        static void Main(string[] args)
        {
            var input = Console.ReadLine();

            // not necessary??
            var s = input.ToCharArray();
            
            var count = 0;
            
            var cont = true;

            while (cont)
            {
                var exitthis = false;
                var haveflipped = false;
                for (var i = 0; i < s.Length; i++)
                {
                    if (/*!exitthis && */i<s.Length-1)
                    {
                        if (s[i] == b && s[i + 1] == w)
                        {
                            s[i] = w;
                            s[i + 1] = b;
                            exitthis = true;
                            haveflipped = true;
                            count++;
                        }
                    } else if ((!exitthis) && i == s.Length - 1 && !haveflipped)
                    {
                        cont = false;
                    }
                }
            }

            Console.WriteLine(count);

            Console.ReadKey();

        }
    }
}