#include <iostream>
#include <vector>

using namespace std;

int main()
{
  int k, n, f;
  cin >> k >> n >> f;
  int beans = k * n;

  int beans_req = 0;
  for(int i = 0; i < f; ++i) {
    int tmp;
    cin >> tmp;
    beans_req += tmp;
  }

  int ans = beans - beans_req;
  if(ans >= 0) {
    cout << ans << endl;
  }
  else {
    cout << "-1" << endl;
  }
}