s = gets.strip
count = 0
if s.length == 1
  puts 0
  exit
end

if s.length.odd?
  1.upto((s.length - 1) / 2) do |i|
    count += 1 if s[i - 1] != s[-i]
  end
else
  1.upto(s.length / 2) do |i|
    count += 1 if s[i - 1] != s[-i]
  end
end

puts count