#include <iostream>

using namespace std;

int main() {
	int bean_num, bag_num, fam_num;
	int *olds;
	int olds_sum = 0;

	cin >> bean_num >> bag_num >> fam_num;
	olds = new int[fam_num];

	for (int i = 0; i < fam_num; i++) {
		cin >> olds[i];
		olds_sum += olds[i];
	}

	if (olds_sum > bean_num * bag_num) {
		cout << -1 << endl;
	}
	else {
		cout << (bean_num * bag_num) - olds_sum << endl;
	}

	delete[] olds;

	return 0;
}