#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N = 3e5;
bool b[N + 5];
int cnt, ans, a;
string s;
signed main() {
	cin >> s;
	int len = s.size();
    s = ' ' + s;
	for (int i = len; i >= 1; i--) {
		if (s[i] == 'C') cnt++;
		else if (s[i] == 'B' && cnt) cnt--, ans++, b[i] = 1;
	}
	for (int i = 1; i <= len; i++) {
		if (s[i] == 'A') a++;
		else if (s[i] == 'B' && b[i] == 0 && a) a--, ans++;
	}
	cout << ans;
	return 0;
}