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

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

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

	return 0;
}