#include <bits/stdc++.h>
using namespace std;

int main() {
  int K, N, F;
  cin >> K >> N >> F;
  vector<int> A(F);
  for (int i = 0; i < F; i++)
    cin >> A[i];
  int ans = K * N;
  for (int i = 0; i < F; i++)
    ans -= A[i];
  if (ans < 0)
    ans = -1;
  cout << ans << endl;
}