import java.util.*;

class Main{
	public static void main(String[]$){
		Scanner s=new Scanner(System.in);
		char[]v=s.next().toCharArray();
		int n=v.length,d[]=new int[n+1];
		for(int i=0;i<n;++i)
			d[i+1]=d[i]+(v[i]=='A'?1:-1);
		
		Map<Integer,Integer> l=new HashMap<>(),r=new HashMap<>();
		for(int i=0;i<n+1;++i){
			l.merge(d[i],i,Math::min);
			r.merge(d[i],i,Math::max);
		}
		System.out.println(r.keySet().stream()
				.map(i->r.get(i)-l.getOrDefault(i,1<<20))
				.mapToInt(o->o).max().getAsInt());
	}
}