#include "bits/stdc++.h"
using namespace std;
#define ll long long int
#define rep(i,n) for( int i = 0; i < n; i++ )
#define rrep(i,n) for( int i = n; i >= 0; i-- )
#define REP(i,s,t) for( int i = s; i <= t; i++ )
#define RREP(i,s,t) for( int i = s; i >= t; i-- )
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define INF 2000000000
#define mod 1000000007
#define INF2 1000000000000000000

int main(void)
{
    cin.tie(0);
    ios::sync_with_stdio(false);
	string S; cin >> S;
	int tmp = 0;
	int ans = 0;
	map<int, int> mp;
	mp[0] = 1;
	rep(i, S.length()) {
		if (S[i] == 'A') tmp++;
		else tmp--;
		if (mp[tmp] == 0)
			mp[tmp] = i + 2;
		else
			ans = max(ans, i + 2 - mp[tmp]);
	}
	cout << ans << endl;

    return 0;
}