#include<algorithm>
#include<complex>
#include<ctype.h>
#include<iomanip>
#include<iostream>
#include<fstream>
#include<map>
#include<math.h>
#include<numeric>
#include<queue>
#include<set>
#include<stack>
#include<stdio.h>
#include<string>
#include<string>
#include<vector>

using namespace std;
typedef long long ll;

#define FOR(i,a,b) for(ll i=(a);i<(b);++i)
#define ALL(v) (v).begin(), (v).end()
#define p(s) cout<<(s)<<endl
#define p2(s, t) cout << (s) << " " << (t) << endl
#define br() p("")
#define pn(s) cout << (#s) << " " << (s) << endl
#define p_yes() p("Yes")
#define p_no() p("No")

const ll mod = 1e9 + 7;
const ll inf = 1e18;

template < typename T >
void vprint(T &V){
	for(auto v : V){
    	cout << v << " ";
	}
	cout << endl;
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    // input
    string s;
    cin >> s;
    ll L = s.size();

    vector<ll> A(L);
    
    ll sum = 0;
    for(int i=0; i<L; i++){
        char c = s[i];
        if(c=='A'){
            sum++;
        }else{
            sum--;
        }
        A[i] = sum;
    }

    // leftmost index
    map<ll, ll> mp;
    mp[0] = -1;

    ll max_len = 0;
    FOR(i, 0, L){
        ll v = A[i];
        if(mp.count(v)==0){
            mp[v] = i;
        }else{
            ll diff = i - mp[v];
            max_len = max(max_len, diff);
        }
    }

    p(max_len);

    return 0;
}