int = gets.to_s
half_size = (int.length / 2)
 
front = int[0, half_size]
back = int[half_size, int.length - 1]
 
counts = 0
 
half_size.times do |n|
  if front[n - 1] != back.reverse[n - 1]
    counts += 1
  end
end
 
p counts