s = gets.chomp.split("").map(&:to_s)
len = s.length
count = len / 2 - 1
i_max = len - 1

ans = 0
0.upto(count) do |i|
  j = i_max - i
  if s[i] != s[j]
    ans += 1
  end
end
if len == 1
  ans = 0
end

p ans