#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define llong long long

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

    vector<int> pos(2*s.length()+1, s.length());
    pos[s.length()] = 0;
    int index = s.length();
    int result = 0;
    rep(i, s.length()) {
        if(s[i] == 'A') index++;
        else index--;
        result = max(result, i+1-pos[index]);
        pos[index] = min(pos[index], i+1);
    }

    cout << result << "\n";
}