#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <deque>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>

#define p(s) cout<<(s)<<endl
#define REP(i,n,N) for(int i=n;i<N;i++)
#define RREP(i,n,N) for(int i=N-1;i>=n;i--)
#define CK(n,a,b) ((a)<=(n)&&(n)<(b))
#define F first
#define S second
typedef long long ll;
using namespace std;
const int inf=1e9+7;

string s;
int ans,cnt;
map<int,int> m;
int sum[200010];
int main(){
    cin>>s;
    m[0]=0;
    REP(i,0,s.size()){
        if(s[i]=='A') cnt++;
        else cnt--;
        sum[i+1]=cnt;
        if(m[cnt]==0&&cnt!=0){
            m[cnt]=i+1;
        }
    }
    RREP(i,0,s.size()+1){

        ans=max(ans, i-m[sum[i]]);

    }
    p(ans);
    return 0;
}