def getlist():
	return list(map(int, input().split()))

#処理内容
def main():
	K, N, F = getlist()
	A = getlist()
	mame = K * N
	for i in A:
		mame -= i

	if mame < 0:
		print(-1)
	else:
		print(mame)



if __name__ == '__main__':
	main()