#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <math.h>
#include <cmath>
#include <limits.h>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <functional>
#include <stdio.h>
using namespace std;

long long MOD = 1000000007;

int A[200001*2] = {0};

int main() {
    string S;
    cin >> S;
    int N = S.length();
    
    int a = 200000;
    int ans = 0;
    for ( int i = 1; i <= N; i++ ) {
        if ( S[i-1] == 'A' ) { a++; }
        else { a--; }

        if ( A[a] ) {
            ans = max( ans, i-A[a] );            
        }
        else {
            A[a] = i;
        }
    }
    cout << ans << endl;
    return 0;
}