#include<bits/stdc++.h>
#define rep(i, x) for(ll i = 0; i < x; i++)
#define rep2(i, x) for(ll i = 1; i <= x; i++)
#define all(a) (a).begin(),(a).end()
using ll = long long;
using ld = long double;
const ll INF = 10000000000000000;
using namespace std;
signed main() {
	ll k, n, a, sum = 0; cin >> k >> n >> a;
	rep(i, a) {
		ll b; cin >> b; sum += b;
	}
	cout << max((ll)-1, k * n - sum) << endl;
	return 0;
}