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 Python (3.4.3) (judge verdict: Accepted)

import numpy as np

S = input()
N = len(S)

w_num_arr = np.zeros(N)
b_num_arr = np.zeros(N)


if S[-1] == "W":
    w_num_arr[-1] = 1

for i in range(N-1-1, -1, -1):
    if S[i] == "W":
        w_num_arr[i] = w_num_arr[i+1] + 1
    else:
        w_num_arr[i] = w_num_arr[i+1]

ans = 0
for i in range(N):
    if S[i] == "B":
        ans += int(w_num_arr[i])

print(ans)