// UUID: e3d85b22-2117-4018-a4cd-2af158eae1c8
#include <bits/stdc++.h>
using namespace std;

int main() {
	int longest = 0;
	string s;cin >> s;
	vector<int> str;
	vector<int> p(s.size() + 1);
	for (int i = 1;i <= s.size();i++) {
		if (s[i - 1] == 'A') p[i] = (p[i - 1] + 1);
		else p[i] = (p[i - 1] - 1);
	}
	vector<int> first;
	for (int i = 0;i < p.size();i++) {
		if (-p[i] > first.size()) {
			first.push_back(i - 1);
		}
	}
	for (int i = 0;i < s.size() + 1;i++) {
		if (p[i] >= 0) longest = max(longest, i);
		else longest = max(longest, i - first[-p[i] - 1] - 1);
	}
	cout << longest;
}