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.Linq;

namespace AGC115_A
{
    class Program
    {
        static void Main(string[] args)
        {
            var s = Console.ReadLine();
            var newS = string.Empty;

            var c = 0;
            var cnt = 0;
            while (true)
            {
                foreach (var i in Enumerable.Range(0, s.Length - 1))
                {
                    if (s[i] == 'B' & s[i + 1] == 'W')
                    {
                        newS = s.Substring(0, i) + "WB" + s.Substring(i + 2, s.Length - i - 2);
                        s = newS;
                        c++;
                        cnt++;
                    }
                }
                if (c == 0) break;
                else c = 0;
            }

            Console.WriteLine(cnt);
        }
    }
}