#include <bits/stdc++.h>
using namespace std;

int k, n, f, a;

int main()
{
	cin >> k >> n >> f;
	n *= k;

	for (int i = 0; i < f; i++) {
		cin >> a;
		n -= a;
	}

	if (n >= 0)
		cout << n << endl;
	else
		cout << -1 << endl;
}