#include <bits/stdc++.h>

using namespace std;

#define DBG cerr << '!' << endl;
#define REP(i,n) for(ll (i) = (0);(i) < (n);++i)
#define rep(i,s,g) for(ll (i) = (s);(i) < (g);++i)
#define rrep(i,s,g) for(ll (i) = (s);i >= (g);--(i))
#define PB push_back
#define MP make_pair
#define FI first
#define SE second
#define SHOW1d(v,n) {for(int W = 0;W < (n);W++)cerr << v[W] << ' ';cerr << endl << endl;}
#define SHOW2d(v,i,j) {for(int aaa = 0;aaa < i;aaa++){for(int bbb = 0;bbb < j;bbb++)cerr << v[aaa][bbb] << ' ';cerr << endl;}cerr << endl;}
#define ALL(v) v.begin(),v.end()
#define Decimal fixed<<setprecision(10)
#define INF 1000000000
#define MOD 1000000007
#define PI 3.14159265359

typedef long long ll;
typedef pair<ll,ll> P;

int num[222222];
bool ok[222222];

int main()
{
	string str;
	cin >> str;
	
	int a = 0;
	
	REP(i,str.size()){
		if(i != 0){
			num[i] = num[i-1];
		}
		if(str[i] == 'A'){
			a++;
			num[i]++;
			ok[i] = true;
		}
	}
//	SHOW1d(num,str.size());
//	SHOW1d(ok,str.size());

	for(int i = min(a,((int)str.size() - a));i > 0;i--){
	//	cout << i << endl;
		for(int j = 2*i-1;j < str.size();j++){
	//		cout << j << ' ' << j-2*i+1 << endl;
			if((num[j]-num[j-2*i+1] == i&&!(ok[j - 2*i+1]))||(num[j] - num[j - 2*i+1] == i-1 && ok[j - 2*i+1])){
				cout << 2*i << endl;
				return 0;
			}
		}
	}
	
	cout << 0 << endl;
	
	return 0;
}