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