#include "bits/stdc++.h"

using namespace std;

using ll = long long;
using ld = long double;
using P = pair<int, int>;
constexpr ld EPS = 1e-12;
constexpr int INF = numeric_limits<int>::max() / 2;
constexpr int MOD = 1e9 + 7;

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);

    int res = 0;
    string s;
    cin >> s;
    int n = s.length();
    map<int, int> mp;
    mp[0] = 0;
    int a = 0, b = 0;
    for (int i = 0; i < n; i++)
    {
        if (s[i] == 'A')
            a++;
        else
            b++;

        if (mp.find(a - b) != mp.end())
        {
            res = max(res, a + b - mp[a - b]);
        }
        else
        {
            mp[a - b] = a + b;
        }
    }
    cout << res << endl;
}