#include<bits/stdc++.h>
using namespace std;
#ifdef LOCAL_DEBUG
  #include "LOCAL_DEBUG.hpp"
#endif
#define int long long

signed main(){

  string s; cin >> s;

  int n = s.size();
  vector<int> dp(n+1, 0);
  map<int, int> mp;
  for(int i = 0; i < n; i++){
    if(s[i] == 'A') dp[i+1] = dp[i] + 1;
    if(s[i] == 'B') dp[i+1] = dp[i] - 1;
    mp[dp[i+1]] = i;
  }

  int ans = 0;
  for(int i = 0; i <= n; i++){
    ans = max(ans, mp[dp[i]] - i + 1);
  }
  if(ans == 1) ans = 0;
  cout << ans << endl;

  return 0;
}