#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); i++)
using namespace std;
using ll = long long;
using pii = pair<int, int>;

int main() {
  int k, n, f;
  cin >> k >> n >> f;
  vector<int> a(f);
  rep(i, f) cin >> a[i];
  int s = 0;
  rep(i, f) s += a[i];
  int ans = k * n - s;
  if (ans < 0) ans = -1;
  cout << ans << endl;
  return 0;
}