#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <iomanip>
#include <cmath>
#include <functional>
using namespace std;
#define diff(x,y) ((x > y) ? (x - y) : (y - x))
#define us(x) (x > 0) ? x : x * -1
#define lli long long int

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

	return 0;
}