#include <stdio.h>
#include <string.h>
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <functional>
#include <map>
#include <iomanip>
#include <math.h> 
#include <stack>
#include <queue>
#include <bitset>
#include <cstdlib>
#include <tuple>
#include <cctype>
#include <ctype.h>
#include <set>
#include <sstream>
#include <time.h>
using namespace std;
//#define int long long
#define rep(i,s,n) for(int i = s;i<n;i++)
#define repe(i,s,n) for(int i = s;i<=n;i++)
#define rrep(i,s,n) for(int i = (n)-1;i>=(s);i--)
#define all(v) (v).begin(),(v).end()
#define pb push_back
#define fi first
#define se second
#define chmin(a,b) a=min((a),(b))
#define chmax(a,b) a=max((a),(b))
#define l1 list[index]
#define l2 list[index - 1]
#define l3 list[index + 1]
#define iif(i,j) ((i<0 && j<0) || (i>0 && j>0)) ? true : false 
typedef long long ll;
typedef pair<int, int>pint;
typedef vector<int>vint;
typedef vector<pint>vpint;
typedef pair<pint, int> P1;
typedef pair<int, pint> P2;
typedef pair<pint, pint> PP;
static const ll maxLL = (ll)1 << 62;
const ll MOD = 1000000007;
const ll INF = 1e18;
const double PI = 3.14159265359;
int ca[12] = { 31,28,31,30,31,30,31,31,30,31,30,31 };



signed main() {
	string s;
	//int n, k;
	vector<vector<int>>list(200000, vector<int>(2));
	int l, cnt = 0, maxn = 0;
	int n, n2, sum = 0;


	cin >> s;

	int num = s.length();

	for (int i = 0, aa=0, bb=0; i < s.length(); i++) {
		if (s[i] == 'A')aa++;
		if (s[i] == 'B')bb++;
		list[i][0] = aa;
		list[i][1] = bb;
	}





	for (int i = num - 1; i >= 0; i--) {

		for (int j = 0; j < s.length() && j + i < s.length(); j++) {

			//int a = 0, b = 0;

			int numa = list[j + i][0] - list[j][0];
			int numb = list[j + i][1] - list[j][1];

			if (s[j] == 'A')numa++;
			if (s[j] == 'B')numb++;

			/*for (int k = j; k < 
			num + j && k < s.length(); k++) {
				if (s[k] == 'A')a++;
				if (s[k] == 'B')b++;
			}*/

			if (numa == numb)maxn = max(maxn, numa + numb);
		}

		
	}
	


	cout << maxn << endl;




	getchar();
	getchar();
	return 0;
}