#include <iostream>
#include <sstream>
#include <vector>
#include <algorithm>
#include <string>
#include <functional>
#include <numeric>
#include <ctype.h>
#include <cstdlib>
#include <string.h>
#include <cmath>
#include <cstdio>
#include <map>
#include <queue>
#include <utility>

using namespace std;

const int INF = 1000000007;
const double PI = acos(-1.0);


int main()
{
	int k, n, f;
	int beans = 0;
	vector<int> peo;
	cin >> k >> n >> f;
	
	beans = k * n;
	for (int i = 0; i < f; i++)
	{
		int input;
		cin >> input;
		peo.push_back(input);
	}

	int ans = beans - accumulate(peo.begin(), peo.end(), 0);

	cout << max(ans, -1) << endl;
	
}