#include <bits/stdc++.h>
using namespace std;
int main(){
  string S;
  cin >> S;
  int N = S.size();
  vector<int> s(N + 1);
  s[0] = 0;
  for (int i = 0; i < N; i++){
    if (S[i] == 'A'){
      s[i + 1] = s[i] + 1;
    } else {
      s[i + 1] = s[i] - 1;
    }
  }
  map<int, int> mp;
  int ans = 0;
  for (int i = 0; i <= N; i++){
    if (mp.count(s[i])){
      ans = max(ans, i - mp[s[i]]);
    } else {
      mp[s[i]] = i;
    }
  }
  cout << ans << endl;
}