#include <bits/stdc++.h>

using namespace std;

int main(void) {
  int k, n, f;
  cin >> k >> n >> f;

  int sum = 0;
  for (int i = 0; i < f; i++) {
    int a;
    cin >> a;
    sum += a;
  }

  cout << (sum <= k * n ? k * n - sum : -1) << endl;

  return 0;
}