# frozen_string_literal: true

C = 100_000

def solve
  c = Array.new(2 * C, [nil, nil])
  c[C] = [C-1, C-1]
  d = C
  S.each_with_index do |s, i|
    s ? (d += 1) : (d -= 1)
    #pp d
    if c[d][0].nil?
      c[d] = [C + i, C + i]
    else
      c[d][1] = C + i
    end
  end
  #pp c.select{_1&&_2}
  c.select { _1 && _2 }.map { (_1 - _2).abs }.max
end

S = gets.chomp.chars.map { _1 == 'A' }

puts solve