#include <bits/stdc++.h>
using namespace std;
 
#define fi first
#define se second
#define pb push_back
using vi = vector <int>;
using ll = long long;
using pii = pair <int, int>;
//~ const ll mod = 998244353;
const ll mod = 1e9 + 7;
ll qpow(ll a, ll b, ll m = mod) { ll r = 1, t = a;
	for(; b; b /= 2) { if(b & 1) r = r * t % m; t = t * t % m; } return r; }

const int N = 2e5 + 11;
int s[N];

int main() {
	ios :: sync_with_stdio(false);
	
	string str; cin >> str;
	int n = str.size();
	for(int i = 0; i < n; i ++)
		s[i + 1] = s[i] + (str[i] == 'A' ? 1 : -1);
	
	int ans = 0;
	map <int, int> mp;
	for(int i = 0; i <= n; i ++) {
		if(mp.find(s[i]) != mp.end())
			ans = max(ans, i - mp[s[i]]);
		
		if(mp.find(s[i]) == mp.end())
			mp[s[i]] = i;
	}
	cout << ans << '\n';
}