#include <bits/stdc++.h>

using namespace std;

#define REP(i,a,b) for(int i=a;i<(int)b;i++)
#define rep(i,n) REP(i,0,n)
#define all(c) (c).begin(), (c).end()
#define zero(a) memset(a, 0, sizeof a)
#define minus(a) memset(a, -1, sizeof a)
#define watch(a) { std::cout << #a << " = " << a << "\n"; }
template<class T1, class T2> inline bool minimize(T1 &a, T2 b) { return b < a && (a = b, 1); }
template<class T1, class T2> inline bool maximize(T1 &a, T2 b) { return a < b && (a = b, 1); }
template<class T, class V> istream& operator>> (istream& ist, pair<T, V>& p) { return ist >> p.first >> p.second; }
template<class T> ostream& operator<< (ostream& ost, pair<T, T>& p) { return ost << p.first << ", " << p.second; }
template<class T> istream& operator>> (istream& ist, vector<T>& vs) { for(auto& e: vs) ist >> e; return ist; }

typedef long long ll;
int const inf = INT_MAX / 2;

int main() {
  string S; cin >> S;
  int const N = S.size();
  vector<int> A(N + 1), B(N + 1);
  int max_len = 0;
  rep(i, N) {
    A[i + 1] += A[i] + (S[i] == 'A');
    B[i + 1] += B[i] + (S[i] == 'B');
    auto diff = A[i + 1] - B[i + 1];
    if (diff >= 0) {
      auto iter = lower_bound(A.begin(), A.begin() + i + 1, diff);
      if (iter != A.end() && *iter == diff) {
        max_len = max(max_len, (int)(A.begin() + i + 1 - iter));
      }
    }
    else {
      diff *= -1;
      auto iter = lower_bound(B.begin(), B.begin() + i + 1, diff);
      if (iter != B.end() && *iter == diff) {
//        cout << "p: " << (iter - B.begin()) << ", diff: " << diff
//             << ", len: " << (int)(B.begin() + i + 1 - iter) << "\n";
        max_len = max(max_len, (int)(B.begin() + i + 1 - iter));
      }
    }
  }
//  rep(i, N + 1) cout << A[i] << " "; cout << "\n";
//  rep(i, N + 1) cout << B[i] << " "; cout << "\n";
  cout << max_len << "\n";
}