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

int main(){
  int k , n,f;
   cin >>  k >> n >> f;
   int check =0;
    vector <int> a (f);
    for (int i =0;i<f; i++){
      cin >> a.at(i);
      check+=a.at(i);
    }
  if(check > k*n){
    cout << -1 << endl;
  }
  else {
    cout << k*n-check << endl;
  }
  }