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

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

  static int n[202020];
  memset(n, 0, sizeof(n));

  map<int, int> st, ed;
  for (int i = -202020; i <= 202020; i++) st[i] = ed[i] = 1e18;

  for (int i = 1; i <= s.size(); i++) {
    if (s[i] == 'A') {
      n[i] = n[i - 1] + 1;
    } else {
      n[i] = n[i - 1] - 1;
    }
  }

  for (int i = 1; i <= s.size(); i++) {
    if (st[n[i]] != 1e18) {
      ed[n[i]] = i;
    } else {
      st[n[i]] = i;
    }
  }

  int ans = 0;
  for (int i = -202020; i <= 202020; i++) {
    if (st[i] != 1e18 && ed[i] != 1e18) ans = max(ans, abs(st[i] - ed[i]));
  }

  cout << ans << endl;
  return 0;
}