#include <bits/stdc++.h>
#define fastIO (cin.tie(0), cout.tie(0), ios::sync_with_stdio(false))
#define precise(i) fixed << setprecision(i)
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
using namespace std;

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

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

  sum = k * n - sum;

  if (sum < 0)
    cout << -1 << '\n';
  else
    cout << sum << '\n';

  return 0;
}