class Yukicoder
  def initialize
    k, n, f = gets.chomp.split(' ').map(&:to_i)
    a = gets.chomp.split(' ').map(&:to_i)

    sum = a.inject(:+)

    if k * n >= sum
      puts k * n - sum
    else
      puts -1
    end
  end
end

Yukicoder.new