#include <iostream>
using namespace std;

int main() {
	int i,sum,left;
	int K, N, F;
	cin >> K >> N >> F;
	int A[F];

	sum = 0;
	left = 0;
	for (i = 1; i <= F; i++)
	{
		cin >> A[i - 1];
		sum += A[i - 1];
	}

	left = K*N - sum;
	if (left < 0) {
		left = -1;
	}
	cout << left << "\n";
	return 0;

}