#include <bits/stdc++.h>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;
typedef unsigned long long ull;


int main(){
  string s;
  cin >> s;

  ll lb = 0, ub = s.size();
  while(ub-lb>1){
    ll m = (lb+ub)/2;

    ll sz = 2*m;
    
    bool flg = false;
    if(sz <= s.size()){
      ll A = 0;
      ll B = 0;
      rep(i,sz){
        if(s[i] == 'A') A++;
        else B++;
      }
      
      if(A == m && B == m) flg = true;
      REP(i,sz,s.size()){
        if(s[i-sz] == 'A') A--;
        else B--;
        if(s[i] == 'A') A++;
        else B++;
        
        if(A == m && B == m) flg = true;
      }
    } 
    if(flg) lb = m;
    else ub = m;
  }

  cout << 2*lb << endl;
  
  return 0;
}