#include <iostream>
#include <algorithm>
using namespace std;

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

	int ans = k * n;
	for (int i = 0; i < f; i++)
	{
		int Ai;
		cin >> Ai;

		ans -= Ai;
	}
	cout << max(-1, ans) << endl;
}