#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

#define rep(i,n) for(int i=0;i<(n);i++)
#define reps(i,f,n) for(int i=(f);i<(n);i++)

int main(int argc, char const *argv[])
{
	string s; cin >> s;
	vector<int> cnt(s.size() + 1);

	int now = 0;
	cnt[0] = 0;
	rep(i, s.size()) {
		if (s[i] == 'A') now++;
		else now--;

		cnt[i + 1] = now;
	}

	map<int, pair<int, int>> ma;
	rep(i, s.size() + 1) {
		if(ma.count(cnt[i])) {
			ma[cnt[i]] = make_pair(ma[cnt[i]].first, i);
		} else {
			ma[cnt[i]] = make_pair(i, i);
		}
	}

	int ans = 0;
	for (auto u : ma) {
		ans = max(ans, u.second.second - u.second.first);
	}
	printf("%d\n", ans);
	return 0;
}