#include <stdio.h>
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <functional>
#include <map>
#include <iomanip>
#include <math.h> 
#include <stack>
#include <queue>
#include <bitset>
#include <cstdlib>
#include <tuple>
#include <cctype>
#include <ctype.h>
#include <set>
#include <sstream>

using namespace std;


int main() {
	int i, j;
	long k, n, f;
	vector<long>a;

	cin >> k >> n >> f;

	long sum = k*n;

	for (i = 0; i < f; i++) {
		long num;
		cin >> num;
		sum -= num;
	}

	if (sum < 0) {
		cout << -1 << endl;
	}
	else {
		cout << sum << endl;
	}











	getchar();
	getchar();
	return 0;
}