#include<iostream>
using namespace std;
int main(void)
{
	int i, k, n, f, ans, a;

	cin >> k >> n >> f;
	ans = k*n;
	for (i = 0; i < f; i++) {
		cin >> a;
		ans -= a;
	}
	if (ans >= 0)cout << ans << endl;
	else cout << -1 << endl;

	return 0;
}