arr = gets.chomp.split('')
cen = arr.count / 2
left = arr.slice(0...cen).reverse
right = arr.count % 2 == 0 ? arr.slice(cen, arr.count) : arr.slice(cen + 1, arr.count)
diff = 0
(0...cen).each do |i|
  diff += 1 if left[i] != right[i]
end
puts diff