#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0;i<(n);++i)
int main() {
	string s; cin >> s;
	int n = s.size();
	int a = 0, ans = 0;
	map<int, int> mp;
	mp[0] = -1;
	rep(i,n) {
		a += s[i] == 'A' ? 1 : -1;
		if (mp.count(a))ans = max(ans, i - mp[a]);
		else mp[a] = i;
	}
	cout << ans << "\n";
	
	return 0;
}