#include <iostream>
using namespace std;
int main(void) {
	int k, n, f, a;
	cin >> k >> n >> f;
	int beans = k * n;
	for (int i = 0; i < f; ++i) {
		cin >> a;
		beans -= a;
	}
	if (beans < 0) beans = -1;
	cout << beans << endl;
	return 0;
}