#include<bits/stdc++.h>

using namespace std;
using ll = long long;
using ld = long double;
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define reps(i,s,n) for(int i=(int)(s);i<(int)(n);i++)
#define allsort(v) sort(v.begin(),v.end())
#define allsortg(v)sort(v.begin(),v.end(),greater<int>());
const ll mod = 1e9 + 7;
const int INF = 1e9;

int main() {
	cin.sync_with_stdio(false);
	string S;
	cin >> S;
	int ans = 0;
	rep(i, S.size()-1) {
		if (S[i] == 'B'&&S[i+1]=='A') {
			swap(S[i], S[i + 1]);
			ans++;
			i = -1;
		}
	}
	cout << ans << endl;
	return 0;
}