#include <iostream>
using namespace std;

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

	cout << "粒の数: ";
	cin >> K;

	cout << "袋の数: ";
	cin >> N;
		
	cout << "家族の数: ";
	cin >> F;

	for (int i = 0; i < F; i++)
	{
		cout << i+1 << "人目の年齢: ";
		cin >> A[i];
	}

	beans = K * N;

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

	if (beans < 0)
	{
		beans = -1;
	}
	
	cout << beans << endl;

	return 0;
}