#include <iostream>
#include <string>

int main() {
    using namespace std;
    string s;
    cin >> s;
    int r = 0;
    int aid, bid;
    do {
        aid = -1, bid = -1;
        for (int i = 0; i < s.length(); i++) {
            if (s[i] == 'A') {
                aid = i;
            } else if (s[i] == 'B') {
                bid = i;
            }
            if (aid != -1 && bid != -1) {
                s[aid] = 'C';
                s.erase(s.begin() + bid);
                break;
            }
        }
    } while(aid != -1 && bid != -1);
    
    for (int i = 0, count = 0; i < s.length(); i++) {
        if (s[i] == 'C') {
            count++;
        } else {
            if (count > r) {
                r = count;
            }
            count = 0;
        }
    }
    cout << r * 2 << endl;
}