#include <bits/stdc++.h>

using namespace std;
using ll = long long;

int main() {
	int K, N, F;
	cin >> K >> N >> F;
	int sumA = 0;
	for (int i = 0; i < F; ++i) {
		int a;
		cin >> a;
		sumA += a;
	}

	cout << max(-1, K * N - sumA) << endl;

	return 0;
}