#include <bits/stdc++.h>

using namespace std;
using ll = long long;

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

    string s;
    cin >> s;

    long n = (long)s.size();
    if (n <= 1)
    {
        cout << "0\n";
        return 0;
    }

    vector<long> vsum(n + 1, 0);

    for (long i = 0; i < n; ++i)
        vsum[i + 1] = s[i] == 'A' ? 1 : -1;

    for (long i = 1; i < n + 1; ++i)
        vsum[i] += vsum[i - 1];

    long res{ 0 };
    map<long, long> m;

    for (long i = 0; i < n + 1; ++i)
    {
        if (!m.count(vsum[i]))
            m[vsum[i]] = i;
        else
            res = max(res, i - m[vsum[i]]);
    }

    cout << res << "\n";

    return 0;
}