#include <bits/stdc++.h>

using namespace std;

using ll = long long;
using ul = unsigned long;

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

    string s;
    cin >> s;

    long n = s.size();

    if (n <= 1)
    {
        cout << "0\n";
        return 0;
    }

    long sum{ n };
    vector<long> vidx(n * 2 + 1, n);
    vidx[sum] = -1;

    long res{ 0 };
    for (long i = 0; i < n; ++i)
    {
        s[i] == 'A' ? ++sum : --sum;

        if (vidx[sum] == n)
            vidx[sum] = i;
        else
            res = max(res, i - vidx[sum]);
    }

    cout << res << "\n";

    return 0;
}