#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;
	char s[200001];
	//char 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();
	int num = strlen(s);

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

	int waru = 4;

	if (num >= 100000) {

		

		int numa = list[num/waru - 1][0];
		int numb = list[num/waru - 1][1];

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

		int i = (num / waru);

		for (int j = 1; j < num && j + (num/waru) < num; j++) {

			if (s[j + i] == 'A') {
				numa++;
			}
			else if (s[j + i] == 'B') {
				numb++;
			}

			if (j != 0 && s[j - 1] == 'A') {
				numa--;
			}
			else if (j != 0 && s[j - 1] == 'B') {
				numb--;

			}


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

		}

	}

	if (maxn != 0) {

		int numa = list[num - 1][0];
		int numb = list[num - 1][1];

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

		bool flag = false;

		for (int i = num - 2; i >= 0; i--) {


			if (s[i + 1] == 'A') {
				numa--;
			}
			else {
				numb--;
			}

			int numa2 = numa;
			int numb2 = numb;

			//for (int j = 1; j < s.length() && j + i < s.length(); j++) {
			for (int j = 1; j < num && j + i < num; j++) {

				if (s[j + i] == 'A') {
					numa2++;
				}
				else if (s[j + i] == 'B') {
					numb2++;
				}

				if (j != 0 && s[j - 1] == 'A') {
					numa2--;
				}
				else if (j != 0 && s[j - 1] == 'B') {
					numb2--;

				}



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

				if (numa2 == numb2) {
					maxn = max(maxn, numa2 + numb2);
					break;
				}

			}

			if (flag == true)break;
			if (maxn != 0)flag = true;

			//if (i % 1000 == 0)cout << "i->" << i << " maxn->" << maxn << endl;

		}

	}
	else {

		if(num>=100000) num /= waru;

		int numa = list[num - 1][0];
		int numb = list[num - 1][1];

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

		bool flag = false;

		for (int i = num - 2; i >= 0; i--) {


			if (s[i + 1] == 'A') {
				numa--;
			}
			else {
				numb--;
			}

			int numa2 = numa;
			int numb2 = numb;

			//for (int j = 1; j < s.length() && j + i < s.length(); j++) {
			for (int j = 1; j < num && j + i < num; j++) {

				if (s[j + i] == 'A') {
					numa2++;
				}
				else if (s[j + i] == 'B') {
					numb2++;
				}

				if (j != 0 && s[j - 1] == 'A') {
					numa2--;
				}
				else if (j != 0 && s[j - 1] == 'B') {
					numb2--;

				}



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

				if (numa2 == numb2) {
					maxn = max(maxn, numa2 + numb2);
					break;
				}

			}

			if (flag == true)break;
			if (maxn != 0)flag = true;

			//if (i % 1000 == 0)cout << "i->" << i << " maxn->" << maxn << endl;

		}

	}

	cout << maxn << endl;




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