#include <bits/stdc++.h>

using namespace std;

int main() {
  string S;
  cin >> S;
  map< int, int > mp;
  mp[0] = -1;
  int sum = 0, ret = 0;
  for(int i = 0; i < S.size(); i++) {
    sum += S[i] == 'A' ? 1 : -1;
    if(mp.count(sum)) ret = max(ret, i - mp[sum]);
    else mp[sum] = i;
  }
  cout << ret << endl;
}