require 'pp'

def gets; ARGF.gets; end

class Solver

  def initialize
  end

  def main
    s = gets.chomp

    h_len = s.length / 2
    if h_len == 0
      puts 0
      return
    end

    ans = 0
    0.upto(h_len-1) do |i|
      if s[i] != s[s.length-i-1]
        ans += 1
      end
    end

    puts ans
  end
end

if __FILE__ == $0
  Solver.new.main
end
