line = gets.chop

def cal(s)
  a = 0
  l = s.length
  if l%2 == 0 then
    for i in 0..(l/2)-1
      if s[i] != s[l-i] then
        a = a+1
      end
    end
    return a
  elsif l%2 == 1 then
    for i in 0..((l-1)/2)-1
      if s[i] != s[l-i] then
        a = a+1
      end
    end
    return a
  end
end
  
p cal(line)