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 sys
input = sys.stdin.readline

def main():
    S = input().rstrip('\n')
    cc = 0
    itr = 0
    for i in range(len(S)):
        if S[i] == 'W':
            itr += 1
            cc += i - (itr - 1)
    S = S[::-1]
    itr = 0
    for i in range(len(S)):
        if S[i] == 'B':
            itr += 1
            cc += i - (itr - 1)
    print(cc//2)
    return

if __name__ == '__main__':
    main()