s = gets.chomp
s = s.split(//).map(&:to_s)

i = 0
j = s.length - 1
h = 0
while j >= i
    #puts "si:#{s[i]}"
    #puts "sj:#{s[j]}"
    h = h + 1 if s[i] != s[j]
    i = i + 1
    j = j - 1
end

puts h