#include <bits/stdc++.h>
#define FOR(i,a,b) for(int i = (a); i < (b); ++i)
#define REP(i,n) FOR(i,0,n)
#define SZ(n) (int)(n).size()
#define ALL(n) (n).begin(), (n).end()
#define MOD % 1000000007
using namespace std;
typedef long long LL;
typedef vector<int> VI;


int main() {
	string s;
	cin >> s;
	if (SZ(s) == 1) cout << 0 << endl;
	else {
		int cnt = 0;
		REP(i, SZ(s)) {
			if (s[i] == 'B' && s[i + 1] == 'A') {
				swap(s[i], s[i + 1]);
				cnt++;
				i = 0;
			}
		}
		cout << cnt << endl;
	}
	return 0;
}