// URL: https://yukicoder.me/problems/no/672

import std.algorithm, std.array, std.container, std.math, std.range, std.typecons, std.string;

version(unittest) {} else
void main()
{
  string S; io.getV(S);

  int[int] h;
  auto a = 0;
  foreach (i, c; S) {
    a += c == 'A' ? +1 : -1;
    h[a] = cast(int)i;
  }

  a = 0;
  auto l = 0;
  foreach (i, c; S) {
    if (a in h && h[a] > i) l = max(l, h[a]-cast(int)i+1);
    a += c == 'A' ? +1 : -1;
  }

  io.put(l);
}

auto io = IO!()();
import std.stdio;
struct IO(string floatFormat = "%.10f", string delimiter = " ", alias IN = stdin, alias OUT = stdout)
{
  import std.conv, std.format, std.meta, std.traits;

  auto getV(T...)(ref T v) { foreach (ref w; v) get(w); }
  auto getA(T)(size_t n, ref T v) if (hasAssignableElements!T)
  { v = new T(n); foreach (ref w; v) get(w); }
  auto getC(T...)(size_t n, ref T v)
  if (allSatisfy!(hasAssignableElements, T))
  {
    foreach (ref w; v) w = new typeof(w)(n);
    foreach (i; 0..n) foreach (ref w; v) get(w[i]);
  }
  auto getM(T)(size_t r, size_t c, ref T v)
  if (hasAssignableElements!T && hasAssignableElements!(ElementType!T))
  {
    v = new T(r); foreach (ref w; v) getA(c, w);
  }
  template getS(E...)
  {
    auto getS(T)(size_t n, ref T v)
    {
      v = new T(n); foreach (ref w; v) foreach (e; E) mixin("get(w."~e~");");
    }
  }

  auto put(bool flush = false, T...)(T v)
  {
    foreach (i, w; v) { putA(w); if (i < v.length-1) OUT.write(delimiter); }
    OUT.writeln;
    static if (flush) OUT.flush();
  }
  auto putB(S, T)(bool c, S t, T f) { if (c) put(t); else put(f); }
  auto putRaw(T...)(T v) { OUT.write(v); OUT.writeln; }

  private
  {
    dchar[] buf;
    auto sp = (new dchar[](0)).splitter;
    void nextLine() { IN.readln(buf); sp = buf.splitter; }
    auto get(T)(ref T v) { if (sp.empty) nextLine(); v = sp.front.to!T; sp.popFront(); }

    auto putR(T)(T v)
    {
      auto w = v;
      while (!w.empty) { putA(w.front); w.popFront(); if (!w.empty) OUT.write(delimiter); }
    }
    auto putA(T)(T v)
    {
      static if (isInputRange!T && !isSomeString!T) putR(v);
      else if (isFloatingPoint!T) OUT.write(format(floatFormat, v));
      else OUT.write(v);
    }
  }
}