#include<iostream>
#include<string>

using namespace std;

int main()
{
	string s; cin >> s;
	int firstA[200005],secondA[200005];
	int firstB[200005],secondB[200005];
	
	int a = 0,b=0;
	
	for(int i=0;i<s.size();i++)
	{
		firstA[i]=-1;
		secondA[i]=-1;
		firstB[i]=-1;
		secondB[i]=-1;		
	}
	
	firstA[0] = 0;
	firstB[0] = 0;
	
	for(int i=0;i<s.size();i++)
	{
		if(s[i] == 'A') a++;
		else b++;
		
		if(a>=b)
		{
			if(firstA[a-b] == -1)firstA[a-b]=i;
			else secondA[a-b] = i;	
		}
		else
		{
			if(firstB[b-a] == -1)firstB[b-a]=i;
			else secondA[b-a] = i;
		}
	}
	
	int ans = 0;
	for(int i=0;i<s.size();i++)
	{
		if(secondA[i] == -1)continue;
		ans = max(ans, secondA[i]-firstA[i]);
	}
	
	for(int i=0;i<s.size();i++)
	{
		if(secondB[i] == -1)continue;
		ans = max(ans, secondB[i]-firstB[i]);
	}
	
	cout << ans << endl;
	
	return 0;
	
}