#include <iostream>
#include <vector>
#include <numeric>
#include <algorithm>
using namespace std;

int main() {
	int k, n, f;
	cin >> k >> n >> f;
	vector<int> a(f);
	for (auto &x : a) cin >> x;
	cout << max(k * n - accumulate(a.begin(), a.end(), 0), -1) << endl;
}