#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;

int main() {
    string s;
    cin >> s;

    int n = s.size();

    vector<pair<int, int>> a(n * 2 + 1, make_pair(n, -1));

    int t = n;
    for (int i = 0;; i++) {
        a[t].first = min(a[t].first, i);
        a[t].second = max(a[t].second, i);
        if (i == n) break;
        t += (s[i] == 'A') * 2 - 1;
    }

    int r = 0;
    for (int j = 0; j <= n * 2; j++) {
        r = max(r, a[j].second - a[j].first);
    }

    cout << r << endl;

    return 0;
}