#include <iostream>
#include <cstdlib>

using std::cout;
using std::cin;
using std::endl;


int main(void)
{
  int k,n,f;
  cin >> k;
  cin >> n;
  cin >> f;

  int sum = 0;
  int old[f];
  for(int i=0;i<f;i++){
    cin >> old[i];
    sum += old[i];
  }

  if ( k*n >= sum )
    cout << k*n - sum << endl;
  else
    cout << "-1" << endl;

  return 0;
}