n=gets.to_s
answer = 0
if n.length%2==1
  n.slice!(n.length/2)
end
first = n[0,n.length/2].split("")
second = n[n.length/2,n.length].reverse!.split("")
first.each_with_index {|a,i|
  if a != second[i]
    answer = answer +1
  end    
}
puts answer
  