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

int main() {
	int K, N, F;
	cin >> K >> N >> F;

	int A;
	int sum = 0;
	for (int i = 0;i < F;i++) {
		cin >> A;
		sum = sum + A;
	}
	int ans = (K*N) - sum;
	if (ans < 0) {
		cout << -1 << endl;
	}
	else {
		cout << ans << endl;
	}

	system("pause");
	return 0;

}