#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define F first
#define S second
#define pii pair<int, int>
#define eb emplace_back
#define all(v) v.begin(), v.end()
#define rep(i, n) for (int i = 0; i < n; ++i)
#define rep3(i, l, n) for (int i = l; i < n; ++i)
#define chmax(a, b) a = (a >= b ? a : b)
#define chmin(a, b) a = (a <= b ? a : b)
#define out(a) cout << a << endl
#define outa(a, n) rep(_, n) cout << a[_] << " "; cout << endl
#define SZ(v) (int)v.size()
#define inf (int)(1e9+7)
#define abs(x) (x >= 0 ? x : -(x))

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int c = 0;
    string s;
    cin >> s;
    while (1) {
        string bs = s;
        rep3(i, 1, SZ(s)) {
            if (s[i] == 'A' && s[i - 1] == 'B') { swap(s[i], s[i - 1]); break; }
        }
        if (bs == s) break;
        c++;
    }
    out(c);
    
}