s = gets.chomp
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
