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# (.NET Core 3.1.201) (judge verdict: Wrong Answer)

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

namespace ProgramingStudying2
{
    class Program
    {
        static void Main(string[] args)
        {
            var s = Console.ReadLine();
            var n = s.Length;
            var ans = 0;
            var Wcounter = 0;
            var dp = new int[n + 1]; // dp[i] = 左からi個目のWの位置
            for(int i = 0; i < n; i++)
            {
                if(s[i] == 'W')
                {
                    Wcounter++;
                    dp[Wcounter] = i;
                    ans += i - Wcounter + 1;
                }
            }
            Console.WriteLine(ans);
        }
    }
}