#include <iostream>
#include <map>
#include <string>

void solve() {
    std::string s;
    std::cin >> s;

    std::map<int, int> pos;
    pos[0] = 0;

    int ans = 0, idx = 0, h = 0;
    for (char c : s) {
        h += (c == 'A' ? 1 : -1);
        ++idx;

        if (pos.count(h)) {
            ans = std::max(ans, idx - pos[h]);
        } else {
            pos[h] = idx;
        }
    }

    std::cout << ans << std::endl;
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}