#include<bits/stdc++.h>
using namespace std;
using Int = long long;
//INSERT ABOVE HERE
signed main(){
  Int k,n,f;
  cin>>k>>n>>f;
  vector<Int> a(f);
  for(Int i=0;i<f;i++) cin>>a[i];
  Int ans=k*n-accumulate(a.begin(),a.end(),0);
  if(ans<0) ans=-1;
  cout<<ans<<endl;
  return 0;
}