#include <iostream>
using namespace std;

int main()
{
	
	int K, N, F, A[1000],beans;

	cin >> K >> N >> F;

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

	beans = K * N;

	for (int i = 0; i < F; i++)
	{
		beans -= A[i];
		if (beans < 0)
		{
			beans = -1;
			break;
		}
	}
	
	cout << beans << endl;

	return 0;
}