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

signed main() {
  ios::sync_with_stdio(false);
  string S;
  cin >> S;
  vector<int> pa(S.size() + 1), pb(S.size() + 1);
  map<int, int> minpos, maxpos;
  minpos[0] = maxpos[0] = 0;
  for (int i = 0; i < S.size(); ++i) {
    pa[i + 1] = pa[i] + (S[i] == 'A');
    pb[i + 1] = pb[i] + (S[i] == 'B');
    int d = pa[i + 1] - pb[i + 1];
    if (minpos.count(d)) minpos[d] = min(minpos[d], i);
    else minpos[d] = i;
    maxpos[d] = max(maxpos[d], i);
  }
  int ans = 0;
  for (auto it = minpos.begin(); it != minpos.end(); ++it) {
    if (maxpos.count(it->first)) {
      ans = max(ans, maxpos[it->first] - it->second);
    }
  }
  cout << ans << endl;
  return 0;
}