#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	string s;
	cin >> s;
	int n = int(s.size());
	vector<int>a(n, 0), b(n, 0);
	if (s[0] == 'A')a[0] = 1;
	else a[0] = -1;
	for (int i = 1; i < n; i++) {
		if (s[i] == 'A')a[i] = a[i - 1] + 1;
		else a[i] = a[i - 1] - 1;
	}
	int ans = 0;
	map<int, int>m;
	for (int i = 0; i < int(s.size()); i++) {
		if (m[a[i]] == 0) {
			m[a[i]] = i;
		}
		else {
			ans = max(ans, i - m[a[i]]);
		}
	}
	cout << ans << endl;
}