#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <utility>

using namespace std;

int main() {
	int k, n ,f, a;
	int total = 0;

	cin >> k >> n >> f;
	for (int i = 0; i < f; i++) {
		cin >> a;
		total += a;
	}

	int ans = k * n - total;
	if (ans < 0) {
		ans = -1;
	}
	cout << ans << endl;
	return 0;
}