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