#include<iostream>

using namespace std;

int main() {
	int K, N, F;
	int a[1000];
	cin >> K >> N >> F;
	for (int i = 0; i < F; i++) {
		cin >> a[i];
	}

	int sum = K * N;
	for (int i = 0; i < F; i++) {
		sum -= a[i];
	}
	if (sum < 0) cout << -1;
	else cout << sum;
	return 0;
}