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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<assert.h>
#define fo(i,a,b) for(int i=(a);i<=(b);++i)
#define fd(i,b,a) for(int i=(b);i>=(a);--i)
#define bfo(i,v,u) for(int i=BB[v],u=B[i][1];i;u=B[i=B[i][0]][1])
#define mset(a,x) memset(a,x,sizeof(a))
#define mcpy(a,b) memcpy(a,b,sizeof(b))
template<typename T> bool chkmin(T &a,const T &b) {return b<a?a=b,1:0;}
template<typename T> bool chkmax(T &a,const T &b) {return b>a?a=b,1:0;}
using namespace std;
typedef long long ll;
int read(){int n=0,p=1;char ch;for(ch=getchar();ch<'0' || ch>'9';ch=getchar())if(ch=='-') p=-1;for(;'0'<=ch && ch<='9';ch=getchar()) n=n*10+ch-'0';return n*p;}
const int N=2e5+5;
int n;
char s[N];
int main()
{
	scanf("%s\n",s+1);n=strlen(s+1);
	int ans=0,t=0;
	fo(i,1,n)
		if(s[i]=='B') ++t;
		else ans+=t;
	printf("%d\n",ans);
	return 0;
}