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

int a[200005];
int b[200005];
int c[200005];
map<int,int> mp;

int main(){
  string s;
  cin >> s;
  int n = s.size();
  for(int i = 1; i <= n; i++){
    if(s[i-1]=='A'){
      a[i] = a[i-1]+1;
      b[i] = b[i-1];
    }else{
      a[i] = a[i-1];
      b[i] = b[i-1]+1;
    }
  }
  int ans = 0;
  for(int i = 1; i <= n; i++) c[i] = a[i]-b[i];
  for(int i = 1; i <= n; i++){
    if(mp[c[i]]==0) mp[c[i]] = i;
    else ans = max(ans, i-mp[c[i]]);
  }
  cout << ans << endl;    
  return 0;
}