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;
		int[]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>left=new HashMap<>(),right=new HashMap<>();
		for(int i=0;i<n+1;++i) {
			left.merge(d[i],i,Math::min);
			right.merge(d[i],i,Math::max);
		}
		
		System.out.println(
				right.keySet().stream()
				.map(i->right.get(i)-left.getOrDefault(i,Integer.MAX_VALUE))
				.mapToInt(o->o)
				.max().getAsInt()
				);
	}
}