#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 mame = K * N;
  for (int i = 0; i < F; i++) {
    mame -= A[i];
  }
  cout << (mame < 0 ? -1 : mame) << endl;
  return 0;
}