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

using namespace std;

typedef pair<long long , long long>p;

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

  cin >> str;

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

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

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

  return (0);
}