#include <bits/stdc++.h>
#define rep(i, a, n) for (int i = a; i < n; i++)
#define repr(i, a, n) for (int i = n - 1; i >= a; i--)
using namespace std;
using ll = long long;
using P = pair<int, int>;
template <typename T> void chmin(T &a, T b) { a = min(a, b); }
template <typename T> void chmax(T &a, T b) { a = max(a, b); }

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);

  int k, n, f;
  cin >> k >> n >> f;
  vector<int> a(f);
  rep(i, 0, f) cin >> a[i];
  int acum = accumulate(a.begin(), a.end(), 0);
  int total = k * n;
  if (acum <= total)
    cout << total - acum << endl;
  else
    cout << -1 << endl;
}