#include <bits/stdc++.h>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;
typedef unsigned long long ull;


int main(){
  string s;
  cin >> s;

  int ret = 0;
  map<int,int> memo;
  memo[0] = 0;
  int level = 0;
  rep(i,s.size()){
    if(s[i] == 'A') level++;
    else level--;

    if(memo.count(level)>0){
      ret = max(ret, i+1-memo[level]);
    }else{
      memo[level] = i+1;
    }
  }

  cout << ret << endl;
  
  return 0;
}