#include <iostream>
#include <vector>
using namespace std;

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