s = gets.chomp.split('')

size = s.size
answer = 0
count = 0
sr = s.reverse

size.times do |i|

	answer += 1 if sr[i] != s[i]

end

puts answer


=begin
#Chomp無しだと\nが代入される
#解けてない
s = gets.chomp.split('')
st = s.length/2
sr = s.reverse

count = 0
ans = 0
st.times do |i|
	if c[i] != sr[count]
	ans += 1
	count += 1
else
	count += 1
end
end

puts ans
=end