#include "bits/stdc++.h"

using namespace std;

using ll = long long int;
using ld = long double;

using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vl = vector<ll>;
using vvl = vector<vl>;
using vvvl = vector<vvl>;
using vd = vector<double>;
using vvd = vector<vd>;
using vs = vector<string>;
using vb = vector<bool>;
using vvb = vector<vb>;

const int INF = (1 << 30) - 1;
const ll INF64 = ((ll)1 << 62) - 1;
const double PI = 3.1415926535897932384626433832795;

const int dx[] = { -1, 0, 1, 0 };
const int dy[] = { 0, 1, 0, -1 };

int gcd(int x, int y) { return y ? gcd(y, x % y) : x; }
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
int lcm(int x, int y) { return x / gcd(x, y) * y; }
ll lcm(ll x, ll y) { return x / gcd(x, y) * y; }

int solve(string& s) {
	int res = 0;
	int n = s.size();

	bool flag = true;
	for (int i = 0; flag; i++) {
		flag = false;
		for (int j = n - 1; j >= i + 1; j--) {
			if (s[j - 1] == 'B' && s[j] == 'A') {
				res++;
				swap(s[j], s[j - 1]);
				flag = true;
			}
		}
	}

	return res;
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);

	string s;
	cin >> s;

	int res = solve(s);
	cout << res << endl;
	return 0;
}