#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0; i < int(n); i++)
#define REP(a, b, n) for(int i = a; i < b; i++)
using lint = long long;

signed main(){
	string s; cin >> s;
	int n = s.size();
	vector<int> data(n + 1, 0);
	rep(i, n){
		if(s[i] == 'A') data[i + 1] = data[i] + 1;
		else data[i + 1] = data[i] - 1;
	}
	int lim_max = *max_element(data.begin(), data.end());
	int lim_min = *min_element(data.begin(), data.end());
	vector<int> memo(lim_max + 1 - lim_min, -1);
	int ans = 0;
	for(int i = 0; i <= n; i++){
		int x = data[i] - lim_min;
		if(memo[x] == -1) memo[x] = i;
		else ans = max(ans, i - memo[x]);
	}
	cout << ans << endl;
}