s = gets.chomp

count = 0

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

  s.chars.group_by(&:itself).values.each do |i|
    unless i.size % 2 == 0
      count += 0.5
    end
  end
  puts count.round
end