#include <algorithm>
#include <cmath>
#include <cstring>
#include <deque>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <string>
#include <unordered_map>
#include <vector>

using namespace std;
using ll = long long;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    string s;
    cin >> s;

    unordered_map<int, deque<int>> m;
    int len = s.size();
    int n = 0;
    for (int i = 0; i < len; i++) {
        if (s[i] == 'A') {
            n++;
        } else {
            n--;
        }
        m[n].push_back(i);
    }

    int ans = 0;
    for (auto p : m) {
        if (p.second.size() > 1) {
            ans = max(ans, p.second.back() - p.second.front());
        }
    }
    if (m[0].size() > 0) {
        ans = max(ans, m[0].back() + 1);
    }

    cout << ans << endl;

    return 0;
}