#include <bits/stdc++.h>
#define rep(i,n) for(int i=(0);i<(n);i++)

using namespace std;

typedef long long ll;

template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }

int n;
vector<int> a, b;

bool cond(int x){
	if(2*x > n) return false;
	for(int i = 0; i + 2*x <= n; i++){
		if((a[i+2*x] - a[i]) == (b[i+2*x] - b[i])) return true;
	}

	return false;
}

// condを満たす最大のxを求める(upper_bound)
// 全てのxが条件を満たさない場合は-1を返す(-1はとりえない値)
int upper_bound_cond(){
	int lb = -1, ub = n;

	while(ub - lb > 1){
		int mid = (lb + ub) / 2;

		if(cond(mid)) lb = mid;   // Answer is in [mid, ub)
		else ub = mid;            // Answer is in [lb, mid)
	}

	// now lb + 1 = ub
	return lb;
}

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);

	string s;
	cin >> s;

	n = s.size();
	a.resize(n+1, 0);
	b.resize(n+1, 0);

	rep(i, n){
		a[i+1] = a[i] + (s[i] == 'A' ? 1 : 0);
		b[i+1] = b[i] + (s[i] == 'B' ? 1 : 0);
	}

	cout << max(2 * upper_bound_cond(), 0) << endl;
}