#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <map>
#include <set>
#include <complex>

using namespace std;

#define REP(i,n)	for(int i=0;i<(n);++i)
#define REPS(i,s,t)	for(int i=(s);i<(t);++i)

#define INF 		2000000007
#define MOD			1000000007

#define MAX 		200005

typedef unsigned int 			uint;
typedef unsigned long long int	ull;
typedef long long int 			ll;

//int dp[MAX];
pair<int,int> dp[MAX];

int main(){
	string str;cin>>str;
	int len = str.size();

	dp[0].first=0;dp[0].second=0;
	REPS(i,1,len+1){
		dp[i].first = dp[i-1].first + (str[i-1]=='A' ? 1 : -1);
		dp[i].second = i;
	}

	sort(dp,dp+len+1);

	int maxl = 0;
	REP(i,len+1){
		auto st = dp[i];
		while(st.first == dp[++i].first)
			;
		maxl = max(maxl,dp[--i].second - st.second);
	}

	cout << maxl << endl;

	return 0;
}