import java.util.Scanner;
public class 最長AB列 {
	public static int getAns(String str) {
		String a="A" ;
		String[] ar = str.split("");
		int k,l;
		int m=0;
		int ans=0;
	outside:for(k=2;k<=ar.length/2;k=k*2) {
				int x=0;
				int y=0;
				for(l=0;l<=ar.length-k;l++) {
					for(int i=0;i<k;i++) {
						if(ar[i+l].equals(a))
							x++;
						else
							y++;
					}
					if(x==y) {
						m=k;
						break;
					}else if(l==ar.length-k)
						break outside;
			}
		}
		return ans=m;
	}

	public static void main(String[] args) {
		Scanner sc=new Scanner(System.in);
		String str = sc.next();
		System.out.print(getAns(str));
	}

}