#!/usr/bin/env ruby

input = gets.chomp.chars

l = input.length
loop do
  i = input.length - 1
  loop do
    break if i < 1

    if input[i] == input[i-1].swapcase
      input[i] = nil
      input[i-1] = nil
      i -= 2
    else
      i -= 1
    end
  end
  input = input.compact
  if input.length == l
    break
  else
    l = input.length
  end
end

puts input.length