#include <bits/stdc++.h>
using namespace std;
struct cww{cww(){ios::sync_with_stdio(false);cin.tie(0);}}star;
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define vv(type, c, m, n, i) vector<vector<type>> c(m, vector<type>(n, i))
#define rep(i,a,n) for(int i=(a), i##_len=(n); i<i##_len; ++i)
#define rrep(i,a,n) for(int i=(a), i##_len=(n); i>i##_len; --i)
#define len(x) ((int)(x).size())
#define mp make_pair
#define eb emplace_back
#define em emplace
#define fi first
#define se second
#define get(a, i) get<i>(a)
#define mt make_tuple
#define mini min_element
#define maxi max_element
#define sum(a, b) accumulate(a, b)
#define chmax(a,b) if(a<b)a=b
#define chmin(a,b) if(a>b)a=b
#define print(t) cout << (t) << "\n";
#define printk(v) rep(i, 0, len(v) - 1) { cout << v[i] << " "; } cout << v.back() << endl;
typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<double> vd;
typedef vector<long long> vll;
typedef vector<string> vs;
typedef pair<int, int> pii;
int a, b, n;
int va[200001];
int vb[200002];
string s;
int solve() {
	cin >> s;
	n = len(s);
	if (n <= 1)
		return 0;
	va[0] = vb[0] = 0;
	rep(i, 1, n + 1) {
		if (s[i - 1] == 'A') {
			++a;
		} else {
			++b;
		}
		va[i] = a;
		vb[i] = b;
	}
	int min_ab = min(a, b);
	for (int i = min_ab  * 2; i > 0; i -= 2) {
		rep(j, i, n + 1) {
			if (va[j] + vb[j - i] == vb[j] + va[j - i])
				return i ;
		}
	}
	return 0;
}
int main() {
	print(solve());
	return 0;
}