S = gets.strip.to_s
M = S.size
cnt = 0
if M%2==0 then
  (0..M/2-1).each do |i|
    if S[i]!=S[M-1-i] then
      cnt += 1
    end
  end
  puts cnt
else
  (0..(M-3)/2).each do |i|
    if S[i]!=S[M-1-i] then
      cnt += 1
    end
  end
  puts cnt
end