#include <algorithm>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <iostream>
#include <map>
#include <memory>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>

using namespace std;

typedef long long ll;

#define sz size()
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define all(c) (c).begin(), (c).end()
#define rep(i,a,b) for(ll i=(a);i<(b);++i)
#define per(i,a,b) for(ll i=(b-1);i>=(a);--i)
#define clr(a, b) memset((a), (b) ,sizeof(a))
#define ctos(c) string(1,c)
#define print(x) cout<<#x<<" = "<<x<<endl;

#define MOD 1000000007

ll d[600000];

int main() {
  string s;
  cin>>s;
  rep(i,0,600000){
    d[i] = 100000000;
  }
  ll index = 300000;
  d[index] = -1;
  rep(i,0,s.sz){
    if(s[i]=='A')index++;
    else index--;
    d[index] = min(d[index],i);
  }
  ll cA = 0;
  ll cB = 0;
  ll mx = 0;
  rep(i,0,s.sz){
    if(s[i]=='A')cA++;
    else cB++;
    if(d[300000+(cA-cB)]!=100000000){
      mx = max(mx,i-d[300000+(cA-cB)]);
    }
  }
  cout << mx << endl;
  return 0;
}