//No.143 豆
#include<iostream>
using namespace std;
int main()
{
	int K, N, F;
	cin >> K >> N >> F;
	int *A = new int[F];
	for (int i = 0; i < F; i++) {
		cin >> A[i];
	}
	int mame = K * N;
	for (int i = 0; i < F; i++) {
		mame -= A[i];
	}
	if (mame < 0)mame = -1;
	cout << mame << endl;
	return 0;
}