#include <bits/stdc++.h>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  string s;
  cin >> s;

  int n = s.length();

  vector<int> f(n+1);
  map<int, int> r;
  r[0] = 0;
  
  int ret = 0;
  REP (i, n) {
    f[i+1] = f[i];
    if (s[i] == 'A') ++f[i+1];
    else --f[i+1];
    if (!r.count(f[i+1]))
      r[f[i+1]] = i+1;
    else
      ret = max(ret, (i+1) - r[f[i+1]]);
  }
  cout << ret << endl;
  
  return 0;
}