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: Wrong Answer)

def main():
        s = input()
        start = forward = backward = 0
        intervals = []
        
        if len(s) > 3:
            for i in range(1, len(s)):
                if s[start] != s[i]:
                    intervals.append((start, i-1, s[start]))
                    if s[start] == 'B':
                        forward += 1
                    else:
                        backward += 1
                    start = i
            intervals.append((start, len(s)-1, s[start]))
            print(len(intervals))
        elif s =='BBW' or 'BWW':
            print(2)
        elif s =='WBW' or 'BWB' or 'BW':
            print(1)
            
        else:
            print(0)
main()