line = gets
def cal(s)
  t = s.reverse
  a = 0
  l = s.length-1
  for i in 0..l
    if s[i] != t[i] then
      for j in 0..l
        if s[j] == s[i] then
          s[j] = t[i]
        end
      end
      a = a+1
    end
  end
  return a
end
  
puts cal(line)/2