#include "bits/stdc++.h"
using namespace std;
using Graph = vector<vector<int>>;
typedef long long ll;
typedef pair<ll, ll> P;
#define rep(i,n) for (ll i = 0; i < (n); ++i)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
const long long mod = 1e9 + 7;

ll gcd(ll a, ll b) {
	return b ? gcd(b, a % b) : a;
}

ll lcm(ll a, ll b) {
	return a / gcd(a, b) * b;
}

int main() {
	ll k, n, f;
	cin >> k >> n >> f;

	ll sum=0;

	rep(i, f) {
		ll num;
		cin >> num;
		sum += num;
	}

	ll ans=k*n-sum;

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