#include<bits/stdc++.h>
using namespace std;
const int N = 4e5 + 5;
int a[N],b[N],pos[N];
string s;
int main(){
    cin >> s;
    int n = s.size();
    s = " " + s;
    for(int i = 1;i <= n;i++){
        a[i] = a[i - 1] + (s[i] == 'A');
        b[i] = b[i - 1] + (s[i] == 'B');
    }
    for(int i = 0;i <= 400000;i++){
        pos[i] = n + 1;
	}
	for(int i = 1;i <= n;i++){
		if(pos[a[i] - b[i] + 200000] == n + 1) pos[a[i] - b[i] + 200000] = i;
	}
	int maxx = 0;
	for(int r = 1;r <= n;r++){
		int x = a[r] - b[r] + 200000;
	    if(pos[x] < r){
	    	maxx = max(maxx,r - pos[x]);
		}
	}
	cout << maxx;
	return 0;
}