#include <bits/stdc++.h>

using namespace std;

int main() {
  int k, n, f, a, total = 0;
  cin >> k >> n >> f;
  for (int i = 0; i < f; i++) {
    cin >> a;
    total += a;
  }

  if (total <= k * n) {
    cout << k * n - total << endl;
  } else {
    cout << -1 << endl;
  }

  return 0;
}