#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 1000000007;
// constexpr int MOD = 998244353;
constexpr int dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1};
constexpr int dy8[] = {1, 1, 0, -1, -1, -1, 0, 1}, dx8[] = {0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U> inline bool chmax(T &a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U> inline bool chmin(T &a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

int main() {
  string s; cin >> s;
  int n = s.length();
  vector<int> sum(n, 0);
  map<int, vector<int>> mp{{0, {-1}}};
  REP(i, n) {
    sum[i] = (i > 0 ? sum[i - 1] : 0) + (s[i] == 'A' ? 1 : -1);
    mp[sum[i]].emplace_back(i);
  }
  int ans = 0;
  REP(i, n) {
    int prev = (i == 0 ? 0 : sum[i - 1]);
    if (mp[prev].size() >= 2) chmax(ans, mp[prev].back() - i + 1);
  }
  cout << ans << '\n';
  return 0;
}