#include <map>
#include <set>
#include <list>
#include <cmath>
#include <ctime>
#include <deque>
#include <queue>
#include <stack>
#include <string>
#include <bitset>
#include <cstdio>
#include <limits>
#include <vector>
#include <climits>
#include <cstring>
#include <cstdlib>
#include <fstream>
#include <numeric>
#include <sstream>
#include <iostream>
#include <algorithm>
#include <iomanip>
#include <cctype>
using namespace std;
typedef long long ll;

int main()
{
	int k, n, f, a, sum1 = 0, sum2 = 0;
	cin >> k >> n >> f;
	sum1 = k*n;
	for (int i = 0; i < f; i++)
	{
		cin >> a;
		sum2 += a;
	}
	if (sum1 - sum2 >= 0)cout << sum1 - sum2 << endl;
	else cout << -1 << endl;
	return 0;
}