a=gets.chomp.chars
b=a.size

c=[]
d=[]
e=''
count=0
if b%2 == 0
  #偶数の処理
  f=b/2
  f.times do |ff|
    c << a[ff]
    d << a[b-ff-1]
    if c[ff] != d[ff]
      d[ff] = c[ff]
      count += 1
    end
  end
else
  #奇数(真ん中除く)処理
  f=b/2
  m=f+1
  b-=1
  a.delete_at(m-1)
  f.times do |ff|
    c << a[ff]
    d << a[b-ff-1]
    if c[ff] != d[ff]
      d[ff] = c[ff]
      count += 1
    end
  end
end

puts count