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 test
{
	public static class Program
	{
		public static void Main()
		{
			var s = Console.ReadLine();
			var n = s.Length;
			var s2 = new List<char>();
			foreach(var c in s) {
				s2.Add(c);
			}
			var count = 0;

			while (true) {
				var flag = true;
				var index = int.MaxValue;
				for(int i = 0; i < n - 1; i++) {
					if(s2[i] == 'B') {
						if(index > i) {
							index = i;
						}

						if(s2[i + 1] == 'W') {
							s2.RemoveAt(i + 1);
							s2.Insert(index, 'W');
							count += i + 1 - index;
							flag = false;
							break;
						}
					}
				}

				if (flag) {
					break;
				}
			}

			Console.WriteLine(count);
		}
	}
}