class Calc0143
  def initialize(args)
    args = args.map { |l| l.chomp.split(/\s+/) }
    @k, @n, @f = args.shift.map(&:to_i)
    @as = args.shift.map(&:to_i)
  end

  def calc
    @k * @n - @as.inject(:+)
  end

  def run
    r = calc
    r < 0 ? -1 : r
  end
end

puts Calc0143.new(STDIN.readlines).run if __FILE__ == $0