s = gets.chomp.chars
n = s.size
if n % 2 == 1
  s1 = s[0 .. n/2 - 1]
  s2 = s[n/2 + 1 .. n - 1].reverse
else
  s1 = s[0 .. n/2 - 1]
  s2 = s[n/2 .. n - 1].reverse
end

cnt = 0
for i in 0 .. n / 2
  if s1[i] != s2[i]
    cnt += 1
  end
end

puts cnt
