s = gets.chomp.split("")

s1, s2 = []

if s.size % 2 == 0 then
	s1 = s[0 ... s.size / 2]
	s2 = s[s.size / 2 .. -1]
else
	s1 = s[0 ... s.size / 2]
	s2 = s[s.size / 2 + 1 .. -1]
end

s2 = s2.reverse

count = 0

s1.each_with_index{ |ss, i|
	if ss != s2[i] then
		count += 1
	end
}

puts count