#include<cstdio>
#include<iostream>
#include<algorithm>
#include<string>
#include<queue>
#include<vector>
#include<functional>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<numeric>
#include<limits>
#include<iterator>

#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define rep(i,n) for(int i=0; i<n; i++)
#define INF (1<<20)

using namespace std;
typedef long long ll;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef pair<ll, char> plc;

int main()
{
	string str;
	cin >> str;

	int b_cnt = 0,swap_cnt = 0;
	for (int i = 0; i < str.size(); i++) {
		if (str[i] == 'B') b_cnt++;
		if (b_cnt && str[i] == 'A')swap_cnt += b_cnt;
	}
	cout << swap_cnt << endl;

	return 0;
}