#include <bits/stdc++.h>
using namespace std;

int main() {
  string S;
  cin >> S;
  int N = S.size() + 1;
  vector<int> V(N);
  for (int i = 0, cnt = 0; i < S.size(); i++) {
    if (S.at(i) == 'A') cnt++, V.at(i + 1) = cnt;
    else cnt--, V.at(i + 1) = cnt;
  }
  vector<int> A(N * 2, -1), B(N * 2, -1);
  for (int i = 0; i < N; i++) {
    if (A.at(V.at(i) + N) == -1) A.at(V.at(i) + N) = i;
    else B.at(V.at(i) + N) = i;
  }
  int ans = 0;
  for (int i = 0; i < N * 2; i++) {
    if (B.at(i) == -1) continue;
    ans = max(ans, B.at(i) - A.at(i));
  }
  cout << ans << "\n";
}