import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author silviase
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        longestABSubString solver = new longestABSubString();
        solver.solve(1, in, out);
        out.close();
    }

    static class longestABSubString {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            String s = in.next();
            int l = s.length();
            int[] dif = new int[l * 2 + 2];
            int now = l;
            Arrays.fill(dif, -1);

            dif[now] = 0;
            int res = 0;
            for (int i = 0; i < l; i++) {
                if (s.charAt(i) == 'A') {
                    now++;
                } else {
                    now--;
                }

                if (dif[now] == -1) {
                    // 更新する
                    dif[now] = i + 1;
                } else {
                    // 比較する
                    res = Math.max(res, i + 1 - dif[now]);
                }
            }
            out.println(res);
        }

    }
}