s = gets.chomp.to_s
w_length = s.length
f_half = nil
l_half = nil
count = 0

if w_length.even?
  f_half = s.slice(0, w_length / 2).split('')
  l_half = s.slice(w_length / 2, w_length / 2).split('')
  f_half.each_index do |idx|
    count += 1 if f_half[idx] != l_half.reverse[idx]
  end
else
  s.slice!(w_length / 2 + 1)
  f_half = s.slice(0, w_length / 2).split('')
  l_half = s.slice(w_length / 2 + 1, w_length / 2).split('')
  f_half.each_index do |idx|
    count += 1 if f_half[idx] != l_half.reverse[idx]
  end
end
puts count