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

int main(){
	long k,n,f;
	cin >> k >> n >> f;
	long total = k * n;
	vector<int> a(f);
	for(auto &x:a) cin >> x;
	for(int i = 0; i < f; i++){
		total -= a[i];
		if(total < 0) break;
	}
	cout << (total ? total : -1) << endl;
	return 0;
}