#include <iostream>
#include <algorithm>
#include <climits>
using namespace std;

int main()
{
	int K,N,F,A;
	cin >> K >> N >> F;
	K *= N;

	for (int i = 0; i < F; ++i)
	{
		cin >> A;
		K -= A;
	}
	
	cout << (K < 0?-1:K);

	return 0;
}