# s = gets
# ans = 0
# for i in 0..s.length do
#   ans += 1 if s[i] != s[s.length - 1 - i]
# end
# puts(ans / 2)

s = gets.chomp
len = s.size
ans = 0
(len/2).times do |i|
 ans += 1 if s[i] != s[-i-1]
end

puts ans
