s = gets.chomp("")

if (s.length)%2 == 0
  half = (s.length)/2
  front = s[0, half].reverse
  back =  s[half, s.length]
  count =0
  half.times do |i|
    if front[i] != back[i]
      count += 1
    end
  end
  puts count
else
  half = (s.length)/2
  front = s[0, half].reverse
  back =  s[half+1, s.length]
  count =0
  half.times do |i|
    if front[i] != back[i]
      count += 1
    end
  end
  puts count
end
