#include <iostream>
using namespace std;
int K, N, F;
int A[100];
int main()
{
	cin >> K >> N >> F;
	for (int i = 0; i < F; i++) cin >> A[i];
	
	int S = 0;
	for (int i = 0; i < F; i++) S += A[i];

	S = K * N - S;
	if (S < 0) S = -1;
	cout << S << endl;

	return 0;
}