#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

int main() {
  int k, n;
  cin >> n >> k;
  int total = k * n;
  int f;
  cin >> f;
  for (int i = 0; i < f; ++i) {
    int a;
    cin >> a;
    total -= a;
  }
  if (total >= 0) cout << total << '\n';
  else puts("-1");
  return 0;
}