#include<iostream>
#include<algorithm>
#include <vector>
#include<cmath>
#include<iomanip>
using namespace std;
typedef long long int lont;
int main() {
	int K, N, F;
	cin >> K >> N >> F;
	vector<int>veca(F);
	for (int ia = 0; ia < F; ia++) {
		cin >> veca.at(ia);
	}
	int sum = 0;
	for (int ib = 0; ib < F; ib++) {
		sum += veca.at(ib);
	}
	int ans = K * N - sum;
	if (ans >= 0) {
		cout << ans << endl;
	}
	else {
		cout << -1 << endl;
	}
}