#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  cin.tie(nullptr)->sync_with_stdio(false);
  string s;
  cin >> s;
  vector cnt(2 * s.size() + 1, vector<int>(0));
  int now = s.size();
  cnt[now].push_back(-1);
  rep(i, s.size()) {
    (s[i] == 'A' ? now++ : now--);
    cnt[now].push_back(i);
  }
  int ans = 0;
  for (auto&& v : cnt) {
    if (v.size() < 2) continue;
    ans = max(ans, v.back() - v.front());
  }
  cout << ans << '\n';
  return 0;
}