s = gets.chomp.to_s

l = s.size

if l.even?
  s_front = s[0..l/2-1]
  s_back = s[l/2..l].reverse
else
  s_front = s[0..l/2-1]
  s_back = s[l/2+1..l].reverse
end

count = 0
s_front.split("").each.with_index do |v,i|
  count += 1 if s_front[i] != s_back[i]
end
p count

