#include <iostream>
using namespace std;
int main(void)
{
	int K, N, F;
	int n = 0;
	int sum = 0;
	int l = 0;
	int a[1000];
	cin >> K >> N >> F;
	for (int i = 0; i < F; i++) {
		cin >> a[n];
		n++;
		l++;
	}
	n = 0;
	for (int flag = 0; flag < l; flag++) {
		sum += a[n];
		n++;
	}
	if
		(N * K < sum) cout << -1 << endl;
	else
		cout << N * K - sum << endl;
}