#include <bits/stdc++.h>

using namespace std;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  string S;
  cin >> S;
  const int N = S.size();
  int cur = 0, res = 0;
  map<int, int> mp;
  mp[0] = 0;
  for (int i = 0; i < N; i++) {
    cur += S[i] == 'A' ? 1 : -1;
    if (mp.find(cur) == mp.end()) {
      mp[cur] = i + 1;
    } else {
      res = max(res, i + 1 - mp[cur]);
    }
  }
  cout << res << '\n';
  return 0;
}