def main():
	k,n,f = map(int,input().split())
	li = list(map(int,input().split()))
	mame = k*n
	nenrei = 0
	for i in range(len(li)):
		nenrei += li[i]
		i += 1

	if(mame >= nenrei):
		print(mame - nenrei)
	else:
		print(-1)

if __name__ == '__main__':
	main()