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: Accepted)

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


class Program {

	const long mod = 1000000007;
	//public static int[] max;


	static void Main(string[] args) {

		char[] cs = new char[] { ' ', ',' };
		//int val = int.Parse(Console.ReadLine());
		//int[] vals = parseAry(Console.ReadLine().Split(cs));
		//string str = Console.ReadLine();
		//string[] strs = Console.ReadLine().Split(cs);

		//string res="";
		//int res = 0; ;
		//int t = int.Parse(Console.ReadLine());
		//string str = Console.ReadLine();
		///int[] vals = parseAry(Console.ReadLine().Split(cs));
		//int n = int.Parse(Console.ReadLine());

		//int num = vals[0];

		long res = 0;
		string str = Console.ReadLine();
		int leftb = -1;
		int leftw = -1;
		for ( int i = 0; i < str.Length; i++ ) {
			if ( str[i] == 'B' ) { leftb = i; break; }
		}
		if ( leftb == -1 ) { Console.WriteLine(0); return; }

		for ( int i = leftb+1; i < str.Length; i++ ) {
			if ( str[i] == 'W' ) {
				res += i - leftb;
				leftb++;			
			}
		}


			Console.WriteLine(res);

		return;
	}


}