#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

#define REP(i,n) for(int i=0;i<n;++i)
#define SORT(name) sort(name.begin(), name.end())
#define ZERO(p) memset(p, 0, sizeof(p))
#define MINUS(p) memset(p, -1, sizeof(p))

#define MOD 1000000007
#define INF 1000000000

string S;
bool check() {
    bool isA = true;
    REP(i, S.length()) {
        if(S[i] == 'B') {
            isA = false;
        }
        if(!isA && S[i] == 'A') { return false; }
    }
    return true;
}
int main()
{
    cin >> S;
    if(S.length() == 1) {
        printf("0\n");
        return 0;
    }
    int ans = 0;
    while(!check()) {
        REP(i, S.length() - 1) {
            if(S[i] == 'B' && S[i+1] == 'A') {
                swap(S[i], S[i+1]);
                ans++;
            }
        }
    }
    printf("%d\n", ans);
    return 0;
}