#include<bits/stdc++.h>
using namespace std;
using ll = long long;
template<class T> using vt = vector<T>;
template<class T> using vvt = vector<vt<T>>;
template<class T> using ttt = tuple<T,T>;
using tii = tuple<int,int>;
using vi = vector<int>;
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define pb push_back
#define mt make_tuple
#define ALL(a) (a).begin(),(a).end()
#define FST first
#define SEC second
#define DEB cerr<<"!"<<endl
#define SHOW(a,b) cerr<<(a)<<" "<<(b)<<endl
#define DIV ll(1e9+7)
const int INF = (INT_MAX/2);
const ll LLINF = (LLONG_MAX/2);
const double eps = 1e-8;
//const double PI = M_PI;  
inline ll pow(ll x,ll n,ll m){ll r=1;while(n>0){if((n&1)==1)r=r*x%m;x=x*x%m;n>>=1;}return r%m;}
inline ll lcm(ll d1, ll d2){return d1 / __gcd(d1, d2) * d2;}
// IT 5000兆　欲しい
/* Coding space */
template<class T>
struct FT{
    vector<T> d;
    FT(size_t n):d(n+1){}
    T sum(size_t i){T s{};for(++i;i>0;i-=i&-i)s+=d[i];return s;}
    T sum(size_t i,size_t j){return i?sum(j)-sum(i-1):sum(j);}
    void add(size_t i,T x){for(++i;i<d.size();i+=i&-i)d[i]+=x;}
};
FT<int> ft[2] = {300000,300000};
string str;
bool check(int x){
  x *= 2;
  for(int i = 0; i + x <= (int)str.size(); i++){
    if(ft[0].sum(i,i + x - 1) == ft[1].sum(i,i + x - 1))
      return false;
  }
  return true;
}

int main(){
  cin >> str;
  rep(i,str.size()){
    if(str[i] == 'A')  ft[0].add(i, 1);
    else ft[1].add(i, 1);
  }
  int top = str.size()/2;
  int bottom = 0;
  while(abs(top - bottom) > 1){
    int mid = (top + bottom) /2;
    if(check(mid))
      top = mid;
    else
      bottom = mid;
  }
  if(check(bottom) == 0) bottom--;
  cout << bottom*2 << endl;
}