#include<bits/stdc++.h>
using namespace std;
const int N = 4e5+7;
int pos[N], a[N], b[N];
int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    string s;
    cin >> s;
    int n = s.size();
    s = ' ' + s;
    for (int i = 1; i <= n; i++)
    {
        if (s[i] == 'A')
            a[i] = a[i - 1] + 1;
        else
            a[i] = a[i - 1];
        if (s[i] == 'B')
            b[i] = b[i - 1] + 1;
        else
            b[i] = b[i - 1];
    }
    int eps = 200000;
    for (int i = 0; i <= 400000; i++)
    {
        pos[i] = n + 1;
    }
    for (int i = 1; i <= n; i++)
    {
        int x = a[i] - b[i] + eps;
        if (pos[x] == n + 1)
            pos[x] = i;
    }
    int ans=0;
    for (int r = 1; r <= n; r++)
    {
        int x = a[r] - b[r] + eps;
        if (pos[x] < r)
            ans = max(ans, r - pos[x]);
    }
    cout << ans;
    return 0;
}