#include<iostream>

int main() 
{
	int k, n, f;

	std::cin >> k >> n >> f;

	int ages = 0;
	int beans = k * n;
	for (int i = 0; i < f; i++)
	{
		int age;
		std::cin >> age;
		ages += age;
	}

	beans -= ages;
	if (beans < 0) 
	{
		std::cout << -1 << std::endl;
	}else 
	{
		std::cout << beans << std::endl;
	}

	return 0;
}