#include <iostream>
#include <math.h>
#include <algorithm>
#include <vector>
using namespace std;

int main(){
    int c,m=0;
    string s;
    cin >> s;
    vector<int> a(2*s.length()+1,-2);
    a[s.length()] = -1;
    c = 0;
    for(int i = 0; i < (int)s.length(); i++) {
        if (s[i] == 'A') {
            c += 1;
        }
        else {
            c -= 1;
        }
        if (a[c + s.length()] == -2) {
            a[c + s.length()] = i;
        }
        else {
            m = max(m,i - a[c + s.length()]);
        }
    }
    cout << m << endl;
    return 0;
}