#include <iostream>
using namespace std;

int main() {
	int k, n, f,i,a,soy;
	cin >> k >> n >> f;
	soy = k * n;
	for (i = 0; i < f; i++) {
		cin >> a;
		soy -= a;
	}
	if (soy < 0)
		cout << -1;
	else
		cout << soy;
}