#include <bits/stdc++.h>
using namespace std;
typedef unsigned long long ul;
typedef signed long long ll;
ul over = 1000000007;

int main(void)
{
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed;
  ll k, n, f;
  cin >> k >> n >> f;
  ll f_sum = 0;
  for (ll i = 0; i < f; ++i) {
    ll a;
    cin >> a;
    f_sum+=a;
  }
  cout << (k*n < f_sum ? -1 : k*n-f_sum) << endl;
  return 0;
}