#include <iostream>
using namespace std;

int main() {
	
	int k,n,f,ans;
	int a[10000];
	ans=0;
	cin>>k>>n>>f;
	for(int i=0;i<f;i++){
		cin>>a[i];
		ans=ans+a[i];
	}
	
	if(k*n>=ans){
		cout<<k*n-ans<<endl;
	}else{
		cout<<"-1"<<endl;
	}
	return 0;
}