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 (GCC 9.2.1) (judge verdict: Wrong Answer)

#include<stdio.h>
#include<string.h>

int main(void){

  char s[200000];
  scanf("%s",s);

  int len = strlen(s);
  int count = 0 , flag = 1;
  int left = 0 , right = len;

  for(int i = 0;i < len;i ++){
    for(int j = right;j > left;j --){
      if(s[i] == 'W'){
        j = 0;
      }
      else if(s[j] == 'W'){
        count += 2;
        s[i] = 'W';
        s[j] = 'B';
        right = j;
        left = i;
      }
    }
  }

  printf("%d\n",count);

  return 0;
}