#include <iostream>

using namespace std;

int main()
{
	int k, n, f;
	cin >> k >> n >> f;
	//vector<int> a(f);
	int oldSum = 0;
	for (int i = 0; i < f; i++)
	{
		int old;
		cin >> old;
		oldSum += old;
		//cin >> a.at(i);
	}

	int allBeans = k * n;
	if (allBeans >= oldSum)
		cout << (allBeans - oldSum) << endl;
	else
		cout << -1 << endl;

	return 0;
}