def max(a,b); a > b ? a : b; end
S = gets.chomp
M = { 0 => 0 }
d = 0

S.chars.each.with_index(1).inject(0) do |s,(c,i)|
  s += (c == 'A' ? 1 : -1)
  if M[s]
    d = max(d, i - M[s])
  else
    M[s] = i
  end
  s
end
puts d