#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <numeric>

using namespace std;
typedef long long int lint;

int main()
{
	int K, N, F, B, ans, tmp;
	vector<int> A;

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

	B = K * N;
	ans = B - accumulate(A.begin(), A.end(), 0);

	cout << (ans < 0 ? -1 : ans) << endl;

	return 0;
}