#include<bits/stdc++.h>
using namespace std;
string s;
int a[200005],b[200005];
int main(){
    cin >> s;
    for(int i = 0;i < s.size();i++){
        if(s[i] == 'A') a[i + 1] = 1;
        else b[i + 1] = 1;
        a[i + 1] += a[i];
        b[i + 1] += b[i];
    }
    int maxx = -1;
    for(int i = 1;i < s.size();i++){
        int t = a[i + 1] - b[i + 1];
        for(int j = 0;j < s.size();j++){
            if(a[j + 1] - b[j + 1] == t) maxx = max(maxx,j - i);
        }
    }
    cout << maxx;
	return 0;
}