#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <map>
#include <set>
#include <stdio.h>
#include <cstring>
#include <queue>
#include <stack>
#include <functional>
#include <math.h>
#include <iomanip>
using namespace std;
#define Would
#define you
const int INF = 999999999;
const int MOD = 1e9 + 7;

int main() {
	int a, b, c, sum = 0, k[1005];
	cin >> a >> b >> c;
	a *= b;
	for (int i = 0; i < c; ++i) {
		cin >> k[i];
		sum += k[i];
	}
	if (sum > a) {
		cout << -1 << endl;
		return 0;
	}
	cout << a - sum << endl;
}