import java.io.BufferedReader; import java.io.IOException; import java.io.InputStream; import java.io.InputStreamReader; import java.io.OutputStream; import java.io.PrintWriter; import java.util.ArrayList; import java.util.Arrays; import java.util.InputMismatchException; import java.util.List; import java.util.StringTokenizer; import java.util.stream.Stream; public class Main { public static void main(String[] args) throws IOException { InputStream inputStream = System.in; OutputStream outputStream = System.out; InputReader in = new InputReader(inputStream); PrintWriter out = new PrintWriter(outputStream); TaskX solver = new TaskX(); solver.solve(1, in, out); out.close(); } static int INF = 1 << 30; static long LINF = 1L << 55; static int MOD = 1000000007; static int[] mh4 = { 0, -1, 1, 0 }; static int[] mw4 = { -1, 0, 0, 1 }; static int[] mh8 = { -1, -1, -1, 0, 0, 1, 1, 1 }; static int[] mw8 = { -1, 0, 1, -1, 1, -1, 0, 1 }; @SuppressWarnings("unchecked") static class TaskX { public void solve(int testNumber, InputReader in, PrintWriter out) { char[] s = in.nextString().toCharArray(); int pad = 200000; int n = s.length; int[] sum = new int[n+1]; for (int i = 0; i < n; i++) { if (s[i] == 'A') { sum[i+1] = 1; } else if (s[i] == 'B') { sum[i+1] = -1; } } Arrays.parallelPrefix(sum, Math::addExact); List[] g = new ArrayList[2*pad]; g = Stream.generate(ArrayList::new).limit(n+pad).toArray(List[]::new); for (int i = 0; i < n+1; i++) { g[sum[i]+pad].add(i); } int ans = 0; for (List list : g) { if (list.size() <= 1) continue; int m = list.size(); ans = Math.max(ans, list.get(m-1) - list.get(0)); } out.println(ans); } } static class InputReader { BufferedReader in; StringTokenizer tok; public String nextString() { while (!tok.hasMoreTokens()) { try { tok = new StringTokenizer(in.readLine(), " "); } catch (IOException e) { throw new InputMismatchException(); } } return tok.nextToken(); } public int nextInt() { return Integer.parseInt(nextString()); } public long nextLong() { return Long.parseLong(nextString()); } public double nextDouble() { return Double.parseDouble(nextString()); } public int[] nextIntArray(int n) { int[] res = new int[n]; for (int i = 0; i < n; i++) { res[i] = nextInt(); } return res; } public int[] nextIntArrayDec(int n) { int[] res = new int[n]; for (int i = 0; i < n; i++) { res[i] = nextInt() - 1; } return res; } public int[] nextIntArray1Index(int n) { int[] res = new int[n + 1]; for (int i = 0; i < n; i++) { res[i + 1] = nextInt(); } return res; } public long[] nextLongArray(int n) { long[] res = new long[n]; for (int i = 0; i < n; i++) { res[i] = nextLong(); } return res; } public long[] nextLongArrayDec(int n) { long[] res = new long[n]; for (int i = 0; i < n; i++) { res[i] = nextLong() - 1; } return res; } public long[] nextLongArray1Index(int n) { long[] res = new long[n + 1]; for (int i = 0; i < n; i++) { res[i + 1] = nextLong(); } return res; } public double[] nextDoubleArray(int n) { double[] res = new double[n]; for (int i = 0; i < n; i++) { res[i] = nextDouble(); } return res; } public InputReader(InputStream inputStream) { in = new BufferedReader(new InputStreamReader(inputStream)); tok = new StringTokenizer(""); } } }