//#include <atcoder/all>
#include <iostream>
#include <numeric>
#include <cmath>
#include <limits>
#include <stdio.h>
#include <iomanip>
#include <string> // string, to_string, stoi
#include <vector> // vector
#include <algorithm> // min, max, swap, sort, reverse, lower_bound, upper_bound
#include <utility> // pair, make_pair
#include <tuple> // tuple, make_tuple
#include <cstdint> // int64_t, int*_t
#include <cstdio> // printf
#include <map> // map
#include <queue> // queue, priority_queue
#include <set> // set
#include <stack> // stack
#include <deque> // deque
#include <unordered_map> // unordered_map
#include <unordered_set> // unordered_set
#include <bitset> // bitset
#include <cctype> // isupper, islower, isdigit, toupper, tolower
using namespace std;
//using namespace atcoder;
using ll = long long;
#define all(A) A.begin(),A.end()
using vll = vector<ll>;
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
using Graph = vector<vector<pair<ll, ll>>>;
ll N,B,K;
ll mod=1e9+7;
vll C;

char tr(char C){
    if(C>'a'){
        return (C-'a'+'A');
    }
   else{
       return (C-'A'+'a');
   }
}

int main(){
   string S;
   cin>>S;
   vll A(400005,-1);
   A[200001]=0;
   ll k=0;
   ll an=0;
   rep(i,S.size()){
    if(S[i]=='A')k++;
    else k--;
    
    if(A[k+200001]!=-1){
        an=max(an,i-A[k+200001]);
    }
    else{
        A[k+200001]=i;
    }
   }
   cout<<an<<endl;
}