#include <bits/stdc++.h>

using namespace std;

using ll = long long;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    string s;
    cin >> s;
    int n = s.length();
    vector<int> c(n + 1, 0);
    map<int, int> l, r;
    l[0] = 0;
    r[0] = 0;
    for (int i = 0; i < n; i++) {
        c[i + 1] = c[i] + (s[i] == 'A' ? 1 : -1);
        if (l.find(c[i + 1]) == l.end()) {
            l[c[i + 1]] = i + 1;
        }
        r[c[i + 1]] = i + 1;
    }

    int ans = 0;
    for (int i = 0; i <= n; i++) {
        ans = max(ans, r[c[i]] - l[c[i]]);
    }
    cout << ans << endl;
    return 0;
}