#include <bits/stdc++.h>
using namespace std;

#define int long long
#define vvi vector<vector<int>>
#define vec vector
#define pq priority_queue
#define all(v) (v).begin(), (v).end()
#define uniqueV(x) sort(x.begin(), x.end()); x.erase(unique(x.begin(), x.end()), x.end());
#define rep(i, n) for (int (i) = (0); (i) < (n); ++(i))
#define repp(i, m, n) for (int (i) = (m); (i) < (n); ++(i))
#define dbg(x) cerr << #x << ": " << x << endl;
#define dbg2(x, y) cerr<<"("<<#x<<", "<<#y<<") = "<<"("<<x<<", "<<y<<")"<<endl;
#define dbg3(x, y, z) cerr<<"("<<#x<<", "<<#y<<", "<<#z<<") = "<<"("<<x<<", "<<y<<", "<<z<<")"<<endl;
#define dbgB(value, size) cerr<<#value<<": "<<bitset<size>(value) << endl;
#define line() cerr << "---------------" << endl;

const int dx[] = {1, -1, 0, 0};
const int dy[] = {0, 0, -1, 1};
const double PI = 3.14159265358979323846;

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a = b; return 1; } return 0; }

template<typename T>
void print1(T begin, T end) {
  while (begin != end) {
    cout << (*begin) << " ";
    *begin++;
  }
  cout << endl;
}

template<typename T>
void print2(T Array, int height, int width) {
  for (int i = 0; i < height; i++) {
    for (int j = 0; j < width; j++) {
      cout << " " << Array[i][j];
    }
    cout << endl;
  }
}

// ------------------------------------------------------------------------------------------

using pii = pair<int, int>;

string s;
int ruiA[200010];
int ruiB[200010];

signed main() {
  cin >> s;
  rep (i, s.size()) {
    if (s[i] == 'A') ruiA[i+1] = 1;
    if (s[i] == 'B') ruiB[i+1] = 1;
  }
  repp (i, 1, s.size()+1) {
    ruiA[i] += ruiA[i-1];
    ruiB[i] += ruiB[i-1];
  }
  // print1(ruiA, ruiA+s.size()+1);
  // print1(ruiB, ruiB+s.size()+1);

  map<int, int> mpA;
  map<int, int> mpB;
  int n = s.size();
  int ans = 0;
  for (int i = 0; i <= n; i++) {
    // 判定
    int a = ruiA[i+1], b = ruiB[i+1];
    if (a > b) {
      int diff = a - b;
      // ruiAでdiffを探す
      if (mpA.count(diff) && ruiB[mpA[diff]] == 0) {
        chmax(ans, b*2);
      }
    } else if (a < b) {
      int diff = b - a;
      // ruiBでdiffを探す
      if (mpB.count(diff) && ruiA[mpB[diff]] == 0) {
        chmax(ans, a*2);
      }
    // dbg2(a, b)
    // dbg2(i, ans)
    // line()
    } else {
      chmax(ans, a*2);
    }


    // map
    if (mpA.count(a) == 0) {
      mpA[a] = i;
    }
    if (mpB.count(b) == 0) {
      mpB[b] = i;
    }
  }

  cout << ans << endl;

  return 0;
}