#include<bits/stdc++.h>
using namespace std;
const int N = 4e5 + 7;
// ??? 
int pos[N], a[N], b[N];
int main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	string s;
	cin >> s;
	int n = s.size();
	s = " " + s;
	for (int i = 1; i <= n; ++ i) {
		a[i] = a[i - 1] + (s[i] == 'A');
		b[i] = b[i-1] + (s[i] == 'B');
	}
	int eps = 200000; // ??? 
	for (int i = 0; i <= 400000; ++ i)
		pos[i] = n + 1;
	for (int i = 1; i <= n; ++ i) {
		int x = a[i] - b[i] + eps;
//		pos[x] = min(pos[x], i);
		if (pos[x] == n + 1) pos[x] = i;
	} 
	int ans = 0;
	// ???????
	for (int r = 1; r <= n; ++ r) {
		int x = a[r] - b[r] + eps;
		if (pos[x] < r) {
			// ?????
			ans = max(ans, r - pos[x]);
		}	 
	} 
	cout << ans;
	return 0;
}