#include <iostream>
using namespace std;
int main(){
  int k,n,f,a,x;
  cin>>k>>n>>f;
  x=k*n;
  for(int i=0; i<f; ++i){
    cin>>a;
    x-=a;
  }
  if(x<0){
    cout<<-1<<endl;
  } else {
    cout<<x<<endl;
  }
  return 0;
}