#include <bits/stdc++.h>
#define FOR(i,bg,ed) for(ll i=(bg);i<(ed);i++)
#define REP(i,n) FOR(i,0,n)
#define MOD 1000000007
#define int long long
using namespace std;
typedef long long ll;
typedef vector<vector<ll>> mat;
const int INF = 1e9;

string S;

bool check(string str)
{
    if (str.find("BA") == string::npos) return true;
    else return false;
}

signed main()
{
    cin >> S;

    int ans = 0;
    while (!check(S)) {
        REP(i,S.length()-1) {
            if (S[i] == 'B' && S[i+1] == 'A') {
                swap(S[i], S[i+1]);
                ans++;
                break;
            }
        }
    }

    cout << ans << endl;
}