#include<bits/stdc++.h>
using namespace std;

int a[100];
int main(){
  int k,n,f;
  cin>>k>>n>>f;
  int sum=0;
  for(int i=0;i<f;i++){cin>>a[i];sum+=a[i];}
  int ma=k*n;
  if(ma-sum<0)cout<<-1<<endl;
  else cout<<ma-sum<<endl;
  return 0;
}