#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#define INF 1000000000

using namespace std;
typedef long long int ll;
typedef pair<int, int> P;

int main()
{
	string s;
	cin>>s;
	int n=s.length();
	int c[200001];
	c[0]=0;
	for(int i=0; i<n; i++){
		if(s[i]=='A'){
			c[i+1]=c[i]+1;
		}else{
			c[i+1]=c[i]-1;
		}
	}
	int m1[400002], m2[400002]={};
	fill(m1, m1+400002, INF);
	for(int i=0; i<=n; i++){
		if(m1[c[i]+200000]>i) m1[c[i]+200000]=i;
		if(m2[c[i]+200000]<i) m2[c[i]+200000]=i;
	}
	int ans=0;
	for(int i=0; i<400002; i++){
		if(m2[i]-m1[i]>ans) ans=m2[i]-m1[i];
	}
	printf("%d\n", ans);
	return 0;
}