#include<cstdio>
#include<cstring>
#include<vector>
#include<queue>
#include<stack>
#include<algorithm>
#include<cmath>
#include<climits>
#include<string>
#include<set>
#include<numeric>
#include<map>
#include<iostream>
using namespace std;
#define rep(i,n) for(int i = 0;i<((int)(n));i++)
#define reg(i,a,b) for(int i = ((int)(a));i<=((int)(b));i++)
#define irep(i,n) for(int i = ((int)(n)-1);i>=0;i--)
#define ireg(i,a,b) for(int i = ((int)(b));i>=((int)(a));i--)
typedef long long ll;
typedef pair<ll, ll> mp;
ll mod = 1e9+7;
ll inf = 1e18;

//WA

int main(void){
	ll sum=0,last[400010],ans=0;
	string S;
	cin>>S;
	reg(i,0,2*S.size())last[i]=-2;
	last[sum+S.size()]=-1;
	rep(i,S.size()){
		if(S[i]=='A'){
			sum++;
		}else{
			sum--;
		}
		if(last[sum+S.size()]!=-2){
			ans=max(ans,i-last[sum+S.size()]);
		}else{
			last[sum+S.size()]=i;
		}
	}
	cout<<ans<<endl;
}