S = gets.to_s.chomp.chars

R = S.dup.reverse

n = S.length
half = n /2

i = 0
cnt = 0

half.times do
  s = S[i]
  r = R[i]
  
  if s == r
    # equal
  else
    r = s
    cnt += 1
  end
  i += 1
end

p cnt