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

#define ll long long

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	int k,n,f;
	cin >> k >> n >> f;
	vector<int> a(f);
	for (int i = 0; i < f; i++) cin >> a[i];
	int sum = accumulate(a.begin(),a.end(),0);
	if (n*k >= sum) cout << n*k - sum << endl;
	else cout << -1 << endl;
	return 0;
}