n = gets.chomp
n = n.split("")
l = n.count/2
if n.count%2==1
  k = n[0..l-1]
  m = n[l+1..n.count-1].reverse!
  count = 0
  i = 0
  for i in 0..l-1 do
    if k[i] != m[i]
      count += 1
    end
    i += 1
  end
  p count
else
  k = n[0..l-1]
  m = n[l..n.count-1].reverse!
  count = 0
  i = 0
  for i in 0..l-1 do
    if k[i] != m[i]
      count += 1
    end
    i += 1
  end
  p count
end
