x = gets.chomp.split("")
if x.size == 1
  puts 0
  exit
end
left_half = left_half = x[0..x.size/2 - 1]
mid = 0
if x.size.even?
  mid = x.size/2 - 1 + 1
else
  mid = x.size/2 - 1 + 2
end
right_half = x[mid..-1].reverse

res = 0
left_half.each_with_index do |i, idx|
  res += 1 unless right_half[idx] == i
end

puts res