#include <iostream>
using namespace std;

int main(void)
{
	int K, N, F;
	cin >> K >> N >> F;
	int t;
	int sum = 0;
	for(int i = 0; i < F; i++)
	{
		cin >> t;
		sum += t;
	}

	if (K * N >= sum)
		cout << K * N - sum << endl;
	else
		cout << -1 << endl;

	return 0;
}