#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
const int INF = 1e9;
const ll LINF = 1e18;
template<class S,class T> ostream& operator << (ostream& out,const pair<S,T>& o){ out << "(" << o.first << "," << o.second << ")"; return out; }
template<class T> ostream& operator << (ostream& out,const vector<T> V){ for(int i = 0; i < V.size(); i++){ out << V[i]; if(i!=V.size()-1) out << " ";} return out; }
template<class T> ostream& operator << (ostream& out,const vector<vector<T> > Mat){ for(int i = 0; i < Mat.size(); i++) { if(i != 0) out << endl; out << Mat[i];} return out; }
template<class S,class T> ostream& operator << (ostream& out,const map<S,T> mp){ out << "{ "; for(auto it = mp.begin(); it != mp.end(); it++){ out << it->first << ":" << it->second; if(mp.size()-1 != distance(mp.begin(),it)) out << ", "; } out << " }"; return out; }

/*
 <url:>
 問題文============================================================
 =================================================================
 解説=============================================================
 ================================================================
 */

ll solve(){
    ll res = 0;
    string S; cin >> S;
    vector<ll> cusumA(S.length()+1,0),cusumB(S.length()+1,0);
    for(int i = 0; i < S.length();i++){
        cusumA[i+1] = cusumA[i] + (S[i] == 'A'?1:0);
        cusumB[i+1] = cusumB[i] + (S[i] == 'A'?0:1);
    }
    ll l = 0, r = S.length();
    ll Q = 100000;
    srand((unsigned int)time(NULL));
    while(Q--){
        while(r >= l){
            ll cntA = cusumA[r] - cusumA[l];
            ll cntB = cusumB[r] - cusumB[l];
            if(cntA == cntB){ res = max(res,r-l); break;}
            if(cntA > cntB){
                if(S[l] == 'A'){
                    l++;
                    continue;
                }
                if(S[r-1]=='A'){
                    r--;
                    continue;
                }
                if(rand()%2){
                    l++;
                }else{
                    r--;
                }
            }else{
                if(S[l] == 'B'){
                    l++;
                    continue;
                }
                if(S[r-1]=='B'){
                    r--;
                    continue;
                }
                if(rand()%2){
                    l++;
                }else{
                    r--;
                }
            }
        }
    }
    return res;
}
int main(void) {
    cin.tie(0); ios_base::sync_with_stdio(false);
    cout << solve() << endl;
    return 0;
}