s = gets
reverse_s = s.reverse
ans = 0
for i in (0..(s.length / 2 - 1)) do
  # puts("s[" + i.to_s + "] = " + s[i])
  # puts("reverse_s[" + i.to_s + "] = " + reverse_s[i])
  if s[i] != reverse_s[i]
    ans += 1
  end
end
puts(ans)
