s=gets.chomp.split("")
leftS = []
rightS = []
sSize = s.size
sSizeHalf = sSize/2
s.delete_at(sSizeHalf) if sSize.odd?
leftS = s[0...sSizeHalf]
rightS = s[sSizeHalf...sSize].reverse
count = 0
sSizeHalf.times do |index|
  count += 1 if leftS[index] != rightS[index]
end
puts count