#include <iostream>
using namespace std;

int main()
{
	int K, N, F, sum;
	int *A;	

	cin >> K >> N >> F;

	A = new int[F];
	sum = K * N;

	for (int i = 0; i < F; i++)
	{
		cin >> A[i];
		sum -= A[i];
	}

	if (sum < 0)
	{
		cout << -1;
	}
	else
	{
		cout << sum;
	}

	delete[] A;
}