#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 = N * K;
  for (int i = 0; i < F; i++){
    ans -= A[i];
  }
  cout << max(ans, -1) << endl;
}