#include<iostream>
#include<string>
#include<string.h>
#include<algorithm>
#include<stdio.h>
#include<cmath>
#include<vector>
#include<utility>
#include<queue>
#include<stack>
#include<stdlib.h>
#define FOR(i, a, b) for(int i=(a);i<=(b);i++)
#define RFOR(i, a, b) for(int i=(a);i>=(b);i--)
#define MOD 1000000007

using namespace std;


int main(void) {
	int k, n, f;
	int a[101];
	int total = 0;
	cin >> k >> n >> f;
	FOR(i, 1, f) {
		cin >> a[i];
		total += a[i];
	}
	if (n*k - total < 0) {
		cout << -1 << endl;
	}
	else {
		cout << n*k - total << endl;
	}
	return 0;
}