#include<iostream>

using namespace std;

int main(){

  int k,n,f,to=0,m;
  cin >> k >> n >> f;
  int a[f];

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

  m = k * n;

  if(m < to)
      cout << -1 << endl;
  else
    cout << m- to <<endl;

  return 0;
}