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

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