#include <bits/stdc++.h>
#define INF 2000000000000000000
#define ll long long
#define pll pair<ll, ll>
using namespace std;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  ll K, N, F;
  cin >> K >> N >> F;
  ll ans = K * N;
  for (ll i = 0; i < F; ++i) {
    ll a;
    cin >> a;
    ans -= a;
  }
  if (ans < 0) {
    cout << -1 << "\n";
  }
  else {
    cout << ans << "\n";
  }
}