#include <bits/stdc++.h>
typedef long long ll;
const int INF = 1e9,MOD = 1e9+7,ohara = 1e6;
const ll LINF = 1e18;
using namespace std;

#define rep(i,n) for(ll (i)=0;(i)<(int)(n);(i)++)
#define rrep(i,a,b) for(ll i=(a);i<(b);i++)
#define rrrep(i,a,b) for(ll i=(a);i>=(b);i--)
#define all(v) (v).begin(), (v).end()
#define pb(q) push_back(q)
#define Size(n) (n).size()
#define Cout(x) cout<<(x)<<endl

ll n,cnt=1,ans=0,b,c,d,cmp,cmpp,m,h,w,x,y,sum=0,pos,dataa[ohara],datab[ohara],coua=0,coub=0;
int dy[]={1,0,-1,0};
int dx[]={0,1,0,-1};
//int dy[]={-1,0,1,-1,1,-1,0,1};
//int dx[]={-1,-1,-1,0,0,1,1,1};
string alph("abcdefghijklmnopqrstuvwxyz"),s;
bool a;
struct edge{int to,cost;};

//-------------------------↓↓↓↓↓↓------------------------

int main(void){
       cin.tie(0);
    ios::sync_with_stdio(false);
      
    cin>>s;
    if(s[0]=='A')a=true;
    else a=false;
    rep(i,Size(s)){
        if(i==0)continue;
        if(a&&s[i]=='B')a=false,dataa[coua++]=cnt,cnt=1;
        else if(!a&&s[i]=='A')a=true,datab[coub++]=cnt,cnt=1;
        else cnt++;
    }
    if(a)dataa[coua++]=cnt;
    else datab[coub++]=cnt;
    rep(i,max(coua,coub)){
        cnt=min(dataa[i],datab[i]);
        cnt=max(dataa[i],datab[max(i-1,0LL)]);
        cnt=max(dataa[i],datab[max(i+1,coub-1)]);
        ans=max(ans,cnt*2);
    }
    Cout(ans);
    return 0;
}