import java.io.InputStream;
import java.io.PrintWriter;
import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.PriorityQueue;
import java.util.Scanner;
import java.util.Set;
import java.util.Stack;
import java.util.TreeMap;
import java.util.TreeSet;

import static java.util.Comparator.*;

public class Main {
    public static void main(String[] args) {
        PrintWriter out = new PrintWriter(System.out);
        Solver solver = new Solver(System.in, out);
        solver.solve();
        out.close();
    }
}
class Solver {
	Scanner sc;
	PrintWriter out;
    public Solver(InputStream in, PrintWriter out) {
    	sc = new Scanner(in);
    	this.out = out;
    }
    // ==================================================================
    Map<Integer, Integer> map = new HashMap<>();
    public void solve() {
    	char[] C = sc.next().toCharArray();
    	int ans = 0, a = 0, b = 0, no;
    	Integer wk;
    	map.put(0, 0);
    	for (int i = 0; i < C.length; i++) {
			if(C[i] == 'A')	a++;
			else			b++;
			wk = map.get(a - b);
			if(wk == null)	map.put(a - b, i+1);
			else {
				ans = Math.max(ans, i - wk + 1);
			}
		}
    	out.println(ans);
    }
}