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 5.4.1) (judge verdict: Accepted)

#include "stdio.h"
#include "string.h"
#include "math.h"

int main(int argc, char const *argv[]) {
  char S[300000];
  scanf("%s", S);
  int N=strlen(S);

  int i;
  int w=0;
  long long int ans=0;
  for (i = 0; i < N; i++) {
    if (S[i]=='W') {
      ans=ans+i-w;
      w++;
    }
  }
  if (w==N||w==0) {
    ans=0;
  }
  printf("%lld",ans);
  return 0;
}