s = gets.chomp.split('')

first = []
second = []

if s.count%2 == 0
	(0..s.count-1).each do |i|
		if i < s.count/2
			first << s[i]
		else
			second << s[i]
		end
	end
else
	(0..s.count-1).each do |i|
		if i < s.count/2
			first << s[i]
		elsif i == s.count/2
			first << s[i]
			second << s[i]
		else
			second << s[i]
		end
	end
end

i = 0
count = 0

second.reverse!

while i < first.count
	if first[i] != second[i]
		count += 1
	end
	i += 1
end

puts count