r_input = list(int(i) for i in input().split())
family_age = list(int(i) for i in input().split())

eat_num = 0
all_bean = r_input[0] * r_input[1]

for i in family_age:
	eat_num += i

if all_bean >= eat_num:
	print(all_bean - eat_num)
else:
	print(-1)