import std.algorithm, std.container, std.conv, std.math, std.range, std.typecons, std.stdio, std.string;

void readV(T...)(ref T t){auto r=readln.splitter;foreach(ref v;t){v=r.front.to!(typeof(v));r.popFront;}}

void main()
{
  string s; readV(s);

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

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

  writeln(l);
}