#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < (n);i++)
#define sz(x) int(x.size())
typedef long long ll;
typedef pair<int,int> P;

int main(){
  string s;
  cin >> s;
  int n = sz(s);
  map<int,int> mp;
  mp[0] = 1;
  int res = 0;
  int cnt = 0;
  rep(i,n) {
    if (s[i] == 'A') cnt++;
    else cnt--;

    if (mp.count(cnt)) res = max(res, i - mp[cnt]);
    else mp[cnt] = i;
  }
  cout << res << endl;
  return 0;
}