#include <bits/stdc++.h>
using namespace std;

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

    int N = S.size();
    vector<int> Min(2 * N + 1, 1e9);
    vector<int> Max(2 * N + 1);
    int A = 0;
    int B = 0;
    for (int i = 0; i < N; i++)
    {
        if (S[i] == 'A')
        {
            A++;
        }
        else
        {
            B++;
        }
        Min[A - B + N] = min(Min[A - B + N], A + B);
        Max[A - B + N] = max(Max[A - B + N], A + B);
    }
    int ans = 0;
    for (int i = 0; i < 2 * N + 1; i++)
    {
        ans = max(ans, (Max[i] - Min[i]) / 2);
    }
    cout << ans * 2;
}