s = gets.chomp

count = 0

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

  (s.chars.size / 2).floor.times do |i|
    if s[i] != s[-i-1]
      count += 0.5
    end
  end
  puts count.round
end