def abc_147_b
    s = gets.chomp
    if s.length == 1
        puts 0
    else
        m = s.length / 2
        cnt = 0
        for i in 0..(m-1)
            cnt += 1 unless s[i] == s[-1*(i+1)]
        end
        puts cnt
    end
end

abc_147_b