#include <iostream>
#include <map>
using namespace std;
int main()
{
    map<int, int> left;
    string s;
    cin >> s;
    int ans = 0;
    int maxi = 0;
    left[0] = -1;
    for (int i = 0; i < s.size(); i++) {
        ans += (s[i] == 'A' ? 1 : -1);
        if (left.find(ans) == left.end()) {
            left[ans] = i;
        } else {
            maxi = max(maxi, i - left[ans]);
        }
    }
    cout << maxi << endl;

    return 0;
}