input_lines = readlines
knf_arr = input_lines[0].split.map(&:to_i)
k = knf_arr[0]
n = knf_arr[1]
beans = k * n
familyAges_arr = input_lines[1].split.map(&:to_i)
familyAges_arr.each do |value|
	beans -= value
	if beans < 0
		beans = -1
		break
	end
end
puts beans