#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> D(N + 1);
  rep(i, N) {
    D[i + 1] = D[i] + (S[i] == 'A' ? +1 : -1);
  }
  map<int, int> pos;
  rep(i, N + 1) {
    if (pos.find(D[i]) == pos.end()) {
      pos[D[i]] = i;
    }
  }
  int ans = 0;
  rep(i, N + 1) {
    auto left_pos = pos[D[i]];
    ans = max(ans, i - left_pos);
  }
  cout << ans << endl;
}