#include <iostream>
#include <map>
using namespace std;
int main()
{
    map<int, int> left;
    map<int, int> right;
    string s;
    cin >> s;
    int ans = 0;
    left[0] = 0;
    for (int i = 0; i < s.size(); i++) {
        ans += (s[i] == 'A' ? 1 : -1);
        if (left.find(ans) == left.end()) {
            left[ans] = i;
        }
        right[ans] = i;
    }
    ans = 0;
    const int N = s.size();
    for (int i = -N; i < N; i++) {
        ans = max(ans, right[i] - left[i]);
    }
    cout << ans << endl;

    return 0;
}