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

int main() {

	int k, n, f;
	cin >> k >> n >> f;

	// vector<int>	a(f);
	int	count=0;
	int	all = n * k;

	for (int j = 0; j < f; j++) {
		int	tmp;
		cin >> tmp;
		count += tmp;
	}

	if (count <= all)
		cout << all - count << endl;
	else
		cout << -1 << endl;

	return 0;
}