s = gets.chomp

count = 0

if s == s.reverse
  puts 0
else
  if s.size % 2 != 0
    count += 1
  end

  s.chars.each do |i|
    if s.chars.count(i) % 2 != 0
      count += 0.5
    end
  end

  puts count.round
end
