#include <iostream>
#include <utility>
#include <map>
#include <algorithm>

using namespace std;

typedef pair<int , int>p;

int main()
{
  p rui[200001];
  rui[0].first = rui[0].second = 0;
  
  map<int, int> mp;
  string str;
  int ans = 0;

  cin >> str;

  for ( int i = 0; i < str.size(); i++) {
    if (str[i] == 'A' ) {
      rui[i + 1].first = rui[i].first + 1;
    } else {
      rui[i + 1].first = rui[i].first - 1;
    }

    rui[i + 1].second = i + 1;

    if ( mp.count(rui[i + 1].first) == 0 ) {
      mp[rui[i + 1].first] = i + 1;
    } else {
      ans = max(ans, i + 1 - mp[rui[i + 1].first] );
    }
    
  }

  printf("%d\n", ans);

  return (0);
}