#include<iostream>
#include<algorithm>
using namespace std;
using lint=int64_t;

int main()
{
	int K,N,F;
	int sum;

	cin >> K >> N >> F;
	for(int i=0;i<F;i++)
	{
		int A;
		cin >> A;
		sum+=A;
	}

	if(sum<=N*K)
		cout << N*K-sum;
	else
		cout << -1;
	cout << endl;
	return 0;
}