#include <iostream>
#include <string>
#include <algorithm>
#include <cstring>

using namespace std;
const int MAX = 222222;
int initialPoint[MAX*2];
int* p = initialPoint + MAX;

int main() {
	string str;
	cin >> str;
	int cur = 0;
	memset(initialPoint, -1, sizeof(initialPoint));
	p[0] = 0;
	int ans = 0;
	for (int i = 0; i < str.size(); ++i) {
		const char c = str[i];
		if (c == 'A') {
			++cur;
		} else {
			--cur;
		}
		if (p[cur] < 0) {
			p[cur] = i;
		} else {
			if (!cur) ans = max(ans, cur+1);
			else ans = max(ans, i - p[cur]);
		}
	}
	cout << ans << endl;
}