#include<iostream>
using namespace std;
int main()
{
	int k, n, f;
	cin >> k >> n >> f;
	int a, sum = 0;
	for (int i = 0; i < f; i++)
	{
		cin >> a;
		sum += a;
	}

	int ans = k*n - sum;
	if (ans < 0)ans = -1;
	cout << ans << endl;
	return 0;
}