#include <iostream>
using namespace std;

int main()
{
	int K, N, F, A;
	cin >> K >> N >> F;
	int answer = K * N;
	for(int i = 0; i < F; ++i)
	{
		cin >> A;
		answer -= A;
	}

	cout << ((answer >= 0) ? answer : -1) << endl;
	return 0;
}