#include <iostream>
#include <cmath>
using namespace std;

int main(){
	int k, n, f, a;
	cin >> k >> n >> f;
	int sum = k * n;

	while (f-- > 0){
		cin >> a;
		sum -= a;
		if (sum < 0){
			cout << -1 << endl;
			return 0;
		}
	}

	cout << sum << endl;

	return 0;
}